What is the position with the least number of pieces so that every legal move ends the game?

It can end in checkmate or stalemate, and all legal moves must end the game.

Something like this, I guess: lichess.org/analysis/8/8/8/8/k7/8/1q6/K7_w_-_-_0_1?color=white

a position where your in check and taking the attacker is mate (in k+q endgame for example)
= 4 pieces

Question is whether you can stalemate with 3
I don’t think you can do it on the next ply tho

This one :
FEN--> k7/2Q5/8/8/8/1q6/K7/R7 w - - 0 1

I don't understand the question. Every legal move leading to victory/draw/defeat or what?

I guess you're speaking about this: lichess.org/forum/general-chess-discussion/all-the-legal-moves-are-mate?page=1

The question would make more sense if it was about the *most* pieces.

Note that a position as in #2 where one of the players is forced to capture to a situation with insufficient material (e.g. just two kings) doesn't satisfy the condition. This is because of FIDE rule 5.2b:

"The game is drawn when a position has arisen in which neither player can checkmate the opponent’s king with any series of legal moves. The game is said to end in a ‘dead position’. This immediately ends the game, provided that the move producing the position was legal. (See Article 9.6)"

So the position in #2 already _is_ drawn.

This FEN :
8/8/8/8/8/8/8/8

Zero pieces, no legal moves.