lb63640,
Take the dunce cap off and get out of the corner immediately!
No such thing as a stupid question, if they were stupid, we wouldn't ask them.
Citrix can be used on a single server, am sure many of us have test or development environments which run a single server. Single server implies access datastore to me, too, so its easier to set up than an SQL based datastore too.
The key issue when considering this for a production environment is that of performance and reliability. With one server, when the cleaner accidentally unplugs the server to plug in the vacuum, its all over bar the Spanish inquisition afterwards! Even one additional server will mena you can probably sleep better at night.
When I build new farms, i normally work out what the expected user load will be, and then allow 20% extra on top of that, for scheduled maintenance, hardware crashes, increased useage, that sort of thing. All you do by having a single server is make any scheduled maintenance or unexpected crashes a show stopper. If you've got a second server, i'd certainly consider having that running and share the load, rather than just sitting there doing nothing. If you're worried about licencing, then you can always limit the loading per server anyway.
For me personally though, its simply a case of not feeling at all comfortable with just one server running in a production environment. Hell, you've got a Microsoft OS running a Citrix application with USERS accessing the system. Surely its just a matter of time before something goes wrong??!
Cheers,
Chris