Infinitely many. Well, if you have a computer with infinite amount of memory, and you are prepared to wait infinitely long
If not, then it is still not really a simple question to answer. It depends on - again - how much memory your computer has, how long you are willing to wait, but also what kind of system you are looking at. For ATK, which uses numerical orbitals, what matters is not the number of atoms but the number of orbitals, and larger atoms like Au require more orbitals than C, say, so with a given amount of resources you can simulate fewer Au atoms than C.
But a rough number is 1,000 atoms. Above that it gets very heavy, but even below that it can be hard for e.g. heavy elements in a dense configuration.
