You can buy as many dev cards as you have resources for, you can only play one in a turn.
But if thats true the max is simply 12, as soon as you get LA or LR
VP dev cards don't count towards the one a turn limit.
They do for buying them.
I've never heard about the one dev per turn buying limit and I see nothing about this in the official rules.
Sorry, getting confused.
Yes you can buy as many as you like, but when you get to 10 VP you win still, so if you are on 9 VP, you buy one VP card, then you win.
This assumes you are not a moron and count to 10 of course.
Victory Point Cards (Yellow Frame)✹
You must keep Victory Point Cards hidden. You may only
reveal them during your turn and when you are sure that you
have 10 victory points—that is, to win the game. Of course,
you can reveal them after the end of the game when someone
else wins. You may play any number of Victory Point Cards
during your turn, even during the turn you purchase them.