how much erotica in an erotic thriller?

The answer is, it depends.

Some publishers demand sex every chapter. Some will just rate it lower in heat and take it with less.

My personal rule of thumb, if there isn't at least sex on average every three chapters, you're probably writing a genre book with sex in it, not erotica. The shorter the work... the more sex you have to pack into it. If I write a 30k novella then 15k of it is likely to be sex of some form (don't always have to do the full bump and grind). In an 85k novel probably 20-30k of that will be devoted to sex.

I don't often see the term erotic thriller. What sort of publisher were you aiming for?

Something I've found in my own experiance is that sex and erotic aren't always the same thing. What people find erotic is different, for different people. So I'd say you need enough sex to keep the story going.