You can pay only once each time Key to the City's triggered ability resolves. You can't pay more to draw additional cards.

Key to the City's last ability triggers during your untap step, but it's put onto the stack at the same time as abilities that trigger at the beginning of your upkeep step. Even though Key to the City's ability triggered first, you may order it before or after other abilities you control that are put onto the stack at this time.

Activating Key to the City's ability targeting a creature that has already been blocked won't cause it to become unblocked.

You can activate Key to the City's first ability without targeting any creature.