16- move rule

Hello all, I have a question. 

Recently my friend said that if an enemy king has no pieces, you need to mate him within 16 moves. If you can't, the game will be a draw. 

Is this rules validated to be followed. Please let me know

If that were true, bishop+ knight+ king vs king would be a draw, but it’s not. (In worst possible position, it takes more than 16)

its not a new question but such a rule doesn't exist. but that would be a good excuse to claim a draw when your opponent has failed to checkmate you in a two bishops endgame

There is no such 16 moves rule.
There is a 50 moves rule:
"9.3
The game is drawn, upon a correct claim by a player having the move, if:
9.3.1
he writes his move, which cannot be changed, on his scoresheet and declares to the arbiter his intention to make this move which will result in the last 50 moves by each player having been made without the movement of any pawn and without any capture, or
9.3.2
the last 50 moves by each player have been completed without the movement of any pawn and without any capture."
https://handbook.fide.com/chapter/E012018