with asterisk you can handle a infenite number of user , so 500 users will not be any problem! It is also not that much of a question of your pcÂ´s CPU. Digium says: "Dual Xeon 2.8GHz processors allowed 80 concurrent G.729 calls."
So it is mainly a problem of the codecs you will use and the bandwith of your internet-connection (up- and download) and your own network.
Let us say that your worst case scenario is about 200 simultaneous calls.
A computer with a quad Core should be able zo handle that. The bandwith you need with sip-Protocol and codec “729 a” is about 4.61 Mbps. (you need a license for that codec. It will just cost you ten bucks per channel) If you use codec 711 for best quality, you will need 15.55 Mbps!
Just if you want to get connected with the PSTN directly, you will have to get some hardware (wild-)cards. Then you can use your asterisk server as a Gateway too. But just buy them, when they are recommended for asterisk use.