How many buildings can I buy per turn in Queendomino?
If you look at the rules,


  You must take the following actions, in the outlined order (Note that only actions A and E are mandatory. The rest are optional):


you can see that you can play each action at most 1 times.

For action C (Building): 


  C - (Optional) Construct a Building:You need to have at least 1 “Town”
  square still in construction in order to construct a building, whether
  this is a square that was added to your territory during a previous
  round or the current one.


It clearly states a building, so you can only construct one for action C and because there is only one (or none) action C in your turn, you can only construct one building per turn.