Twenty States Allow Pre-Conviction Pardons
The default rule is that states follows the federal rule that a crime can be pardoned any time after it is committed, but cannot be pardoned before it is committed.
It appears that this is the rule in 20 U.S. states (a compilation of state clemency laws and procedures can be found at this website and a compact chart for all 50 states is here). Admittedly, this evaluation relies on a third party summary that may not capture every fine nuance of the process or exception to the general rule.
Four States Where Pre-Conviction Pardons Are Allowed Don't Actually Grant Them Under A Policy Which Is Actually Followed
This list of 20 states include states where the Governor or Board or both working together, have the power to grant clemency in a very broad array of circumstances, but have adopted policies for how the current Governor or Board will handle applications that are more restrictive than the legal authority that the person issuing pardons has to grant them.
In Indiana, Massachusetts, Minnesota and Nebraska, the pardon power is legally very broad but recent Governors, as a matter of personal pardon power policy, have refused to consider applications for pardons by people who have not completed their sentences many years earlier (5 years in IN; 7 years in MN; 10-15 years depending upon the offense in MA; 3-10 years depending upon the offense in NE).
In addition to these four states, North Carolina's Governor has an informal five year from completion of sentence waiting period. But, North Carolina is not included because in practice, the Governors of North Carolina have granted almost all pardons awarded in cases where the Governor is commuting sentences due to a likelihood of actual innocence of a crime, notwithstanding this policy.
Commutation Of Sentences And Pre-Conviction Pardons Are Very Rare
In practice, however, commutation of a sentence for crimes, or pardons of people who have not completed sentences for their crimes of conviction are very rare in every state, although the frequency with which pardons are granted varies wildly from state to state.
For example, in Alaska, the pardon power is legally very broad, but has been exercised only three times since 1995, while in Pennsylvania the hybrid Governor-Board pardon power is theoretically more narrow but about 150 pardons are granted per year (a rate about 1000 times greater before adjusting for population, and more than 30 times greater after adjusting for population).
The vast majority of pardons are issued to people who have been convicted of a crime and served their sentences and shown good behavior after their release in order to relieve the applicants of the collateral consequences of having a criminal record, such as ineligibility for occupational licenses and loss of gun ownership rights.
Pardons for people who have not been convicted of a crime and commutations of the sentences of people who have been convicted of crimes and are still serving their sentences are extremely rare in every state, and pardons for people who have not been convicted of a crime at the state level are less common than commutations of people who are currently serving sentences for crimes they have been convicted of by courts.
There are probably fewer than twenty such pardons or commutations per year in the United States on average (excluding several cases in which a Governor has commuted the sentence of everyone sentenced to death to life in prison).
The number of pardons of people who have not been convicted of a crime at the state level is probably less than five per year on average in the entire United States - although there are occasional spikes (e.g. in the case of pending prosecutions where serious doubt has been cast on a common source of evidence like a state informant or a crime lab).
Most of the notable instances of pardons of people who have not been convicted of crimes (e.g. President Carter's pardon of Vietnam era draft dodgers) involve categorial pardons of a class of people rather than case by case evaluations of individuals, and resemble a legislative amnesty process to serve a political goal, rather than an individualized quasi-judicial consideration of a particular individual's case in the interests of justice tempered by mercy.
Immunity From Prosecution
On the other hand, even when a state limits the pardon power to persons who have been convicted of a crime in a court of law (which many appear to), there is something almost equivalent to the pardon power for people who have not been convicted of crimes that is routinely used by executive branch DAs (i.e. a grant of immunity from prosecution for a crime, for example, in exchange for testimony or cooperation with an investigation).
This practice is quite common, although so far as I know, there are no comprehensive statistics available regarding immunity from prosecution grants, although there may be some estimates of how many are made in the academic literature.
Governors v. Pardon Boards v. Hybrid Systems
While it doesn't go to the thrust of your question, it is true, however, that while every state has a pardon power, not every state vests that power in the Governor of the state on the federal model.
Many states (e.g. Georgia, Texas and Oklahoma) require the involvement of a Board of Pardons and Paroles (or an equivalent body) to be involved any time that a pardon is sought, sometimes independently of the Governor, and sometimes in coordination with the Governor. Wikipedia states that nine states have Boards of Pardon and Parole or the equivalent with exclusive power over pardons. In the other forty-one U.S. states the pardon power is vested either in both the Governor and a Board, or is vested entirely in a Governor.
The pardon power of the President extends only to offenses
recognizable under federal law. However, the governors of most of the
50 states have the power to grant pardons or reprieves for offenses
under state criminal law. In other states, that power is committed to
an appointed agency or board, or to a board and the governor in some
hybrid arrangement (in some states the agency is merged with that of
the parole board, as in the Oklahoma Pardon and Parole Board).
Nine states in the United States have Boards of Pardons and Paroles
that exclusively grant all state pardons. These states are: Alabama
(Board of Pardons and Paroles), Connecticut (Board of Pardons and
Paroles), Georgia (Board of Pardons and Paroles), Idaho (Commission of
Pardons and Paroles), Minnesota (Board of Pardons), Nebraska (Board of
Pardons), Nevada (Board of Pardon Commissioners), South Carolina
(Board of Probation, Parole and Pardon), and Utah (Board of Pardons
and Parole).
In states that vest the pardon power in part or in full to a Board of Pardons and Paroles, as opposed making it a plenary power of the Governor personally which is not subject to review or limitation as in the federal model, as a practical matter, it is much harder to fit into the Board's bureaucratic process for processing pardon applications when there is not a conviction that has been entered, than it is in the less bureaucratic case when that power is vested solely and personally in the Governor on a plenary basis.
The pardon board process in many states, at least as a practical matter, makes it impossible to obtain a pardon until there has been a conviction and in most cases, also a sentence imposed and sometimes a waiting period after a sentence has been fully served.
For example, many states prohibit applications to the pardon board from being made until a sentence has been completed or until a certain number of years after a sentence has been completed (effectively limiting the power to restoration of civil rights rather than commutation of a sentence, or relief for someone who has not been convicted).
