As I understand it, the only limitation is the number of conversations based on your licence. Extensions and trunks, (and I assume bridges, in that later category), have no limitations for practical purposes, at least none that I have seen reported. Your hardware specs will play a role in limiting size.
Of course, with that many sites, you are going to have to plan out your numbering scheme and outbound rules to try to keep things (dialling) as simple as possible.