"Some"? 29 episodes across TNG, DS9, VOY and ENT. That's even one more TV episode than Jonathan Frakes directed. (although Frakes also directed two movies)
Come to think of it: Star Trek had a bunch of actors directing episodes every now and then. I don't think that many other TV shows did that, didn't they?
I remember a few long running TV shows with cast writing and directing episodes.
Duchovny directed a few XFiles episodes.
Two of the leads directed episodes og SG-1, antihero wrote at least one episode.
Those two are just off the top of my head, but I remember noticing cast doing extra stuff regularly. Probably not too common in the early seasons, but it is a great way for them to branch out.
So I was curious and went to look up actors who directed episodes of their shows and guess who showed up on the list (not in a Star Trek number but still) ... Captain James Tiberius Kirk.