In this rule

Historical Rules · 29 September 2017

Rule 305.2b

Official source text from the 29 September 2017 Rules release

Permalink

A player can’t play a land, for any reason, if the number of lands the player can play this turn is equal to or less than the number of lands he or she has already played this turn. Ignore any part of an effect that instructs a player to do so.

Find this number in the current Rules