How many hydrogens does propyne have?

Propyne has 4 Hydrogens.

The prefix "prop-" indicates three carbons, while the suffix "-yne" indicates a triple bond.

Therefore,

H-C-(triple bond)--C-CH3

Three C-C-C in a row, with a triple bond between either the first two or the last two (it doesn't matter which because the molecule is technically the same, just rotated). There will then be four bonds in order to complete the octet rule and satisfy each C with four bonds.