I've read quite different solutions to this, so to speak. Some stories have one sex-scene blending over into the next. Some regular fantasy novels even are filled with erotic scenes. And some fetish porn stories have no actual sex, but plenty of erotic activites (could possibly be considered sex since they cater to soecific fetishes).
My current WIP is interesting in ths regard. There's 8 sex cenes in it, and 7 of them includes elements that's critical to the main plot. So I don't mind if they go beyond 2-3K words each.
And in other places, as mentioned, 500 words may be just what the doctor ordered.
So write the scenes as you think they should be, and as many as the story needs, and see what you get. If the scenes are good, then your not failing yet.