I'm from a non-EU country, I have received some Job contracts from Germany. Is it possible to sign them all, so in any case, I will not be without a contract if I lose one? What is the legality behind doing that? And what if I want to cancel a contract: do I need to send them my resignation but they still need to accept it. Thanks
Don't do it. It is of course breach of contract when you signed a contract with no intention to fulfil it.
However, you are talking about Germany. German employers take a very dim view of this. While a UK employer would say "good riddance" and do nothing, many German employers would see that as a personal insult. It's something that you just don't do in Germany. There's a good chance that they will do what they can to make you miserable if you do this. For example, inform authorities that you just cancelled your contract which may get a visa cancelled. Or tell the company that you want to start with, which will also take a very dim view of this. Or sue you for damages, not because they want to get the money, but to make you miserable.
On the other hand, if they send you a contract, and you sign it, you have a contract.