A full Starship will require 39 engines, and they will likely lose some during development. Even with a raptor a day that is less than 10 Starships a year. Of course if it is fully reusable and doesn't explode too often even 10 per year will be too much production (unless large-scale Mars colonization missions actually happen ). But it will take time, and for the meantime SpaceX does need to build up a fleet of Starships, and that takes a lot of engines.