Incorporating a little comedy into an episode does not make it a "comedy episode". In order for that to happen, there can be no real danger to our heroes (i.e. "Tribbles", "Action"). I don't think there are a total of ten comedy episodes in TOS.
I didn't say they had to be full comedic episodes. Some of them have a serious plot with some very funny/silly bits within. Certainly Garth was a pretty funny character, up until he really showed his dark side with killing Marta. Kirk's little stint in the pain chair was brief and inconsequential. How on Earth can you say there was nothing humorous in "Plato's Stepchildren", what with all the gags that were played on the crew? Unless you're thinking in real literal terms... in which case there was hardly anything humorous in all of the episodes. Heck, if you go that far, then you could fear the tribbles were on the verge of overrunning the ship and preventing anyone from performing their duties with their furry clutter (putting the ship at risk, especially with the Klingons nearby), or everyone starving because the tribbles consumed all of the food resources. Nothing funny in that.