On Linux, it's 30ish lines, with half of those there to make false.c able to reuse some code. (I know, it's stupid.)
In OpenBSD, it's 3 LoC IIRC.
On at least one OS I worked with, it was 0 bytes long because /bin/sh on an empty file returns true. (I think that was IRIX 4.x.) OTOH, that's slower than the executable from a 3 LoC do-nothing program.