Don't think its something you can i tie a number down to buddy! Numerous variables that could effect the speed/access of the card but you should be ok with 10 on your Uno Server!
You need to bear in mind if you have to do anything to your server then everyone will lose connection, hence why most choose a PC server and link boxes to that, at least your main reciever
isn't tied up that way, for viewing or image/server changes e.t.c!
Pc Server wise depends on your spec again but the basic of Pc's should be ample to run a good server, providing its setup correctly!
Also bare in mind Internet connection for your server (pc or uno) if you or family canning it via downloads or streaming e.t.c this may also have an effect at times!
Most setup, try, test and tweak accordingly to get the best setup!