If I cast Winds of Abandon for its overload cost which says,
Exile each creature you don't control. For each creature exiled this way, its controller searches his or her library for a basic land card. Those players put those cards onto the battlefield tapped, then shuffle their libraries.
and an opponent has 5 creatures, does that opponent search their library five times for 1 basic land or search once for 5 basic lands?
Just once - multiple searches that occur before shuffling are treated as a single search.
701.19h An effect may instruct a player to search a library for one or more cards more than once before instructing a player to shuffle that library. This is the same as a single instruction for that player to search that library for all those cards. The player searches that library only once.
Since Winds of Abandon instructs the player to potentially search their library multiple times for a basic land and then shuffle, it should be treated as searching the library once for multiple basic lands.
