Need OS X Server Unlimited Client?

Hi, I'm just looking to provide web hosting, email and FTP services to about 20 clients (and growing). Will I be able to host ONLY 10 virtual domains, with email and FTP using the 10 Client version of OS X Server? If I would like to host an unlimited number of virtual domains, will I need to buy the unlimited client version of OS X Server?

thanks in advance,
allin