How many successful stars came out of Mad TV?
I would say Key & Peele are the big ones right now. Also Phil LaMarr and Alex Borstein for voice-over stuff. And course Artie Lange is a pretty famous comedy personality as well. And you have people like Ike Barinholtz, Andy Daly, and Arden Myrin have been getting a lot of steady work, even if they're not necessarily household names.
I guess technically you can count people like Taran Killam (SNL) and Simon Helberg (Big Bang Theory) but I don't think most people would associate them with MAD since they were only on there for a very brief time.
Seems rather low compared to SNL which is a shame because this show had a lot of talented people I feel got overlooked by Hollywood because Mad TV was never respected as much as SNL for whatever reason. Stephnie Weir and Michael McDonald were just as funny as the people who came out of SNL from the last 20 years, yet they've been in relative obscurity since Mad TV ended.