Donald Trump is currently the 45th president of the United States. If he is re-elected in November his "number" will not change. If he loses, runs again in 2024 and wins that election, he would become the 47th president (like Grover Cleveland was both 22nd and 24th president). Is their anyway, e. g. by using the line of succession rules (anyone beyond the vice president is not counted as president), that he (or any other president) can be assigned two consecutive "numbers"?
There is no law governing the 'number' of the president. Common sense suggests that a person can't be 45th and 46th; there must be someone in between having the presidency and they will become the 46th president.
In the extremely unlikely scenario that a foreign power occupies the United States this year, eliminates the office of President, and a few years later the US is liberated and Donald Trump is re-elected, I guess people could name him the 45th and 46th President, but it makes no sense to speculate about this.
Actually, as pointed out in another answer, there is no law or official status of which number a president is. We (you, anyone) could say Grover Cleveland only gets counted once, and DJT is the 44th person to be president. We (you, anyone) could say any change in person or any new term gets a new count and DJT is the ...I'm not going bother counting, but say N and is trying to become N+1 also. But N is a lot more than 45 or 46.
There are a variety of parlor tricks available, but none get us to 46.
Presidents before George Washington, during the Articles of Confederation- does not work, because either they don't count at all, or you need to count like 7 or 8 people, not just 1.
Twenty-fifth amendment and the Presidential Succession Act.- Either the new person becomes president #46 (say Pence) and then DJT comes back later and he is 45 and 47, or the new person is temporarily carrying out the duties of president, but is not counted. The temporary duties has happened at least twice- George H.W. Bush and Dick Cheney. So either we count them both, and DJT is now #47, or not at all and he is #45. If we don't count them, and something happens that Pence or Pelosi or whoever takes over temporarily then it does not matter, and if we do count them then DJT is already 47 and anyway he would then be 47 and 49, not consecutive.
I suppose you could come up with some convoluted scenario that we have no president for a short time, and then DJT becomes president again and you say he is 45 and 46, but really that is not possible. The Constitution, the XXVth Amendment, and the Presidential Succession Act ensure that we always have a president, or someone who has officially been recognized as taking the duties temporarily. In the latter case, we either count the person in the numbering or we do not.
The creative answer by @Clint_Eastwood actually gives you something. I'm giving it a +1.
Even if you try to combine these- count Cleveland once, but also count Bush and Cheney- aha! DJT is number 46. Nope, because if you count Cleveland once, then you should count Bush the elder once also under this scheme.
tl;dr Even with creative use of the counting, under the current laws, how we have counted the individual presidents in the past, and historical events, there is no way one person can have two consecutive numbers. If we count differently, he would not be 45 and 46 or 46 and 47. If we count past events differently but in an internally consistent way, there is no way he gets #46.
Trump can sign an executive order recognizing the sovereignty of the Confederacy and legitimizing Jefferson Davis's presidency. He will then be the 45th president in the North and the 46th president in the South. This is similar to how in the United Kingdom, Elizabeth's regnal number is II in England, N. Ireland, and Wales but I in Scotland.