FC5 you're joking right???? good grief. fedora core is a developement distribution. it's sole reason for existing is so that it can make the embarrassing cock-up's on behalf of redhat so they don't have to with rhel. if you think it's ever ever a good idea to use fedora on a noddy home email server you're wrong, but you want to serve DHCP to 20,000 people with it???
there's no maximum limit as to what can be handled in theory, and certainly no reason FC5 couldn't "handle" it, but you need to think a lot more about the overall architecture... i'd suggest have a redundant implementation of rhel 4.4 ES, or the CentOS equivalent if you'd prefer. run a good heartbeat implementation over the top of it and keep all dhcp leases synchronized every few seconds to avoid issues in the event of a failvoer. you should be asking these sorts of questions first, what distro to run second... third even - your choice of hardware should rate higher too.