Regulation game
Bare minimum 4½ innings, 3 outs per inning. 15 outs. So if the home pitcher can get all outs with one pitch, the bare minimum number of pitches is also 15.
The pitcher’s team has to score, of course.
“If the home team (which bats last) is already ahead in the score, and theoretically would not need its half of the fifth inning, then 4½ innings (i.e., the middle of the fifth) is considered an official game. “
Wikipedia, Official game (baseball)
https://en.m.wikipedia.org/wiki/Official_game_(baseball)