SpaceX’s Falcon Heavy used 27 Merlin engines during its first launch in 2018.
Falcon Heavy was built from three Falcon 9-derived first-stage cores. Each core carried nine Merlin 1D engines, producing a total of 27 engines at liftoff. The arrangement gave the rocket substantially greater thrust than a single Falcon 9.
Its maiden flight took place on February 6, 2018, from Kennedy Space Center’s Launch Complex 39A. The payload was Elon Musk’s personal Tesla Roadster, which was placed into a heliocentric orbit.
The number 27 refers to the entire launch vehicle’s first-stage engine cluster. It does not include the single Merlin engine on Falcon Heavy’s upper stage, so the full rocket had 28 Merlin engines in total.