Why is there a limit to how many buildings can built by each player? I was the builder lady lord, using the Undermountain Expansion, and would have been about to build 12+ buildings, but game rules limit each player to 9. What is the reason for this limit?
There are no building limits in Lords of Waterdeep as defined in the Appendix 3: Clarifications section in the rulebook of Scoundrels of Skullport:
Number of Buildings in Play: You are not limited to the ten empty spaces on the game board for Building tiles when putting Buildings into play. Simply place any additional Buildings in a convenient place on or near the board.
Also on the official FAQ for Lords of Waterdeep:
Can more than 10 Buildings be in play?
Yes. The number of empty Building spaces on the board is no limit to the number of Buildings that can be in play during the game.
As for personal building limits, you are in fact limited to the number of building tokens you have, as defined in the official FAQ:
Can you control more Buildings than you have control markers?
No. The number of control markers you have is the maximum number of Buildings you can have in a single game.
This limit is also enforced on the iOS game.