Well it is 5% after 13 living NPCs on the map. the default chance with 0 npc is 100% but going down with every existing NPCs like this:
80%, 64%, 52%, 41%, 33%, 26%, 21%, 17%, 13%
11%, 8%, 7%, 5%, 4%, 3%, 3%, 2%, 2%, 1% etc…
You can see a new NPCs if you travel far from the existing ones because the game “unloads” them so there will be fewer of them. But if you play on the same large area you will stick with your few hiding NPCs. 