In Catan, multiple actions can be taken per turn, so multiple development cards can be drawn. However, I realized that if somebody has 9 VPs and the draw 2 development cards, they could get 11 VPs, but in tournament play where amount of VPs matter, drawing extra VPs would help. But technically, the person would have after drawing only 1 VP card, so the 2nd one was basically drawn after winning the game, which would be technically spending cards and getting an extra victory point after the game was won, so, my question is, should Development card be drawn one at a time to stop extra VPs?
Buying multiple Development Cards at once is a legal play.
The rules state this:
Now you can build. Through building, you can increase your victory points, expand your road network, improve your resource production, and/or buy useful development cards. To build, you must pay specific combinations of resource cards (see the Building Costs Card). Take the appropriate number of roads, settlements, and/or cities from your supply and place them on the game board. Keep development cards hidden in your hand.
(Taken from this answer: In Settlers of Catan, are all actions on the same turn simultaneous?)
So, spending resources can be done in bulk, with no distinction between roads, villages/cities or development cards.
The Rules Almanac, an appendix to the main rules book, has some clarifications and additional limitations to playing Development Cards. These are that you cannot play development cards on the turn you buy them, and can only play one development card per turn. VP cards are an exception to both of these rules however, as you don't play them but reveal them when you get to 10VP.
Although the almanac states that buying your tenth VP ends the game, it also allows for buying in bulk.
The FAQ doesn't exactly clear things up either:
May I continue scoring points after having reached the number of victory points required for a win?
This could make sense in a private tournament where the scores from several games are added up. For individual games and official tournaments, the following applies: You have won as soon as you have reached the required number of victory points on your turn – and then the game is over.
The Catan Studio Tournament rules for national tournaments make clear points above 10 are irrelevant.
2.10 A player has won if they have 10 or more victory points (although only 10 will be counted for the ranking) and it is the player’s turn. If a player does not notice that they have 10 points, the player must wait until the next turn before winning the game (assuming no earlier player wins, and the player still has 10 points).
Note that this also contradicts the accepted wisdom of Winning without Declaring from the standard Catan rules.