At the beginning of your upkeep, if an opponent controls three or more creatures, sacrifice Defense of the Heart, search your library for up to two creature cards, and put those cards onto the battlefield. Then shuffle your library.
If this card is not on the battlefield at the time it resolves, then you can’t sacrifice it. You still perform the search and put any creature cards you find onto the battlefield.
In a multiplayer game, it triggers if at least one opponent controls three or more creatures.
You choose how many cards to put onto the battlefield during the resolution after you look through your library. You do not choose as the ability goes on the stack.
This triggered ability only triggers if your opponent controls 3 or more creatures at the beginning of upkeep. This condition is checked again on resolution.