How much sex depends on the publisher and their target market. I write for an erotic romance publisher, and some of my books don't have any actual sex scenes until half way through the book, because the story I'm telling will carry along quite nicely on UST until then. But there are other publishers where you must have sex by the third chapter, and every couple of chapters after that, and still another group of publishers where there must be sex in every chapter and ideally on the first page.
Pick a book from each a couple of different areas, and read it. Get a feel for what's needed and what you're comfortable with. For example, it may be that you're happy writing very explicit sex, but you need a few chapters to set the story up before you feel comfortable having the characters in bed (or up the wall, or behind the bike shed at work...). In that case, you're not going to feel comfortable writing for Black Lace, but you might be happy writing for one of the erotic romance publishers. Or you don't need it really steamy, but you do want a decent plot, you're into cross-genre, and you like a happy ending. In that case, get one of Tor's paranormal romances and read that to get a feel for whether that's the sort of book you'd like to write.
