So, if i understand you right, what you mean is that there will be collisions and the collisions will be more than two. But even there will be collisions, it is not very likely to happen.
Essentially, yes. In the same way that there are many different private keys which will all lead to the same address and therefore be able to spend the same bitcoin, there are many different inputs to SHA256 which will all give the same output. However, the chances of ever finding such a collision are so astronomically small as to be completely negligible. It is exponentially more likely that someone would guess all your credit card information by just punching in random numbers than they would find a collision with one of your private keys.
But with quantum computers in place... i don't know if i shall worry.
Quantum computers are not some magical device that can solve any and every problem. Quantum computers will be relatively ineffective against hash functions, and it is highly unlikely anyone would use them to try to find a hash collision in order to steal someone's bitcoins. Rather, their main threat against bitcoin would be solving the ECDLP, which would allow the reversal of a known public key in to its paired private key.