From what I've read, they do not enforce a limit. I've definitely seen sites that run 4, 5, 6+ ads. But I always recommend reviewing their guidelines...
I cannot find an exact answer or limitation according to their TOS: However, I would avoid going "overboard" and turning off site visitors.
If you are running any more than 3 ad units, you might want to place them in IFrames. Chitika ads can make websites load alot slower than normal.