Quite actually it can easily hit 20, the real question is who knows of 20 off the top of their head? I own a lot of Diablo-like games, most of which are older and a few newer that are obvious knock-offs.
Dungeon Siege is created by Gas Powered games, which personally I feel are one of the next big powers in the PC gaming world. They have some of the old Blizzard programmers over there and idealists from the Total Annihilation days. I'm sure they'll at some point begin cracking at a MMORPG to compete with WoW, EQ2 and some of those others.
The Boulders Gate series is a great one. It's worth a pick-up for either console or PC and has many spells, detailed colors, dynamic character featuristics including clothes and decent story lines. Controls are fluid and comparable to Diablo and packages of the games can be bought new for $20-30 at retail and game stores.
I'd suggest to stay clear of Dungeon Lords though. I downloaded their demo a few months after their launch and it was sloppy and poor quality. It was just a little while afterwards I found the game for under $10 used but still $60 new at Game Stop. You can actually go on eBay and people aren't even bidding on it for pennies and dollars for shipping. It really says a lot.
For a larger list though, check out this link:
http://en.wikipedia.org/wiki/List_of...-Playing_Games