I knew it was self repeating but didn't know, and haven't experienced multiple legions per turn

There is no way (afaik) to unstation a legion.  What I usually do is create (via projects or generals) as many legions as I want to garrison, and queue them onto the desired planets on the same turn, until no more remain available.  The project will stop repeating if there are no legions available.  Then, once all have been stationed, if I want more for transports, I create more.

Kinda sucks, but that's how the game is at the moment.

I just found a planet with 35 legions on it, so you are wrong.

On the other hand, I tend to go for Research or Alliance victories (Alliance is a bit tougher now) so play a longer game.

EDIT:  the civ with that planet got the "booster artifact" and my 7 legions killed 30 of theirs, so it still wasn't a big deal, just saying that the AI does indeed garrison.

They only had 2 other planets, both with no legions.  It happened to not be their homeworld, either, it was confusing to me.

I ended up not getting it, as after taking the other 2 planets, they surrendered to one of my allies (so still no big deal).

EDIT:  it was the planet closest to the event artifact (the red one, I forget what they call it), so maybe that's why so many legions?  It was only a class 14 with no huge specials.