The Unreal Engine does a lot of additional work on geometry to draw it in a way that is optimal for the particular graphics platform it’s running on.
I’ve seen demo scenes from Epic and on the marketplace with lots of decals, for each little scuff mark and edge trim and footprint. Seems to work fine.
Unless you have measured that decal instance count is a problem, for example by seeing a large difference in frame rate when decals are on versus forced off, on your target system, I wouldn’t worry about it.