A standard SpaceX Falcon 9 rocket’s first stage is powered by nine Merlin engines.
The arrangement is known as an octaweb: eight engines surround a ninth engine in the center. Merlin engines burn rocket-grade kerosene, called RP-1, with liquid oxygen. The nine-engine layout gives Falcon 9 substantial liftoff thrust while allowing the vehicle to continue operating after an individual engine failure.
Falcon 9’s second stage uses one Merlin Vacuum engine, so the rocket has ten Merlin engines in total when both stages are counted. This question refers only to the first stage. The number also differs from Falcon Heavy, whose three first-stage cores collectively provide 27 Merlin engines during launch. Falcon 9’s first stage can return for landing and has been reflown many times, but its engine count remains nine.