I think most people and almost every Tarot reader -- regardless of what they privately believe -- is going to "sell" you Tarot in one of two ways:
1. "oh, it's just a game! Don't take it seriously! Though the cards sometimes speak the truth... for example, my cousin one day was told that [anecdote]" <-- this take irritates me because it's the most dishonest. Be brave and take a stand! Either you believe there's something going on or you don't, don't cowardly hedge your bets.
2. "The cards are magical, there's a system to them, they channel the energies of [explanation]" <-- I bet most people believe this, or in the case of Tarot readers, claim to believe it for the sake of their customers. And it's bs.
No-one really claims "this is just therapy to get you unstuck, you could just as well doodle with a pencil, but let's use these expensive cards/session instead". Well, you are sort of claiming it here, but most people don't. I've never met a Tarot... uh, "fan", who claimed this, for example.
Sorry, I didn't address this. Of course I don't believe in the supernatural or the devil, so my reaction against this is not from a "God fearing" perspective.
I mean "the occult" as in "fringe practices that claim to use the supernatural, spirit world, etc." and I object to it because it's fraudulent and harmful magical thinking. I'm not worried about the "spirit world" itself, because it doesn't exist; I'm just worried about the bullshit.
"Occult" is indeed just a word that means "hidden", but its actual usage has implications.
You are asking how many people use the Tarot cards in this manner. Firstly, the number is non-zero because I am doing it, and I know others who do it. Secondly, as I do not believe the cards are magical, I believe everybody who is getting some meaningful results are using the Tarot cards in this way, even if they are not conscious about it.
I think the question you really want to ask is that how many people believe the carda have some supernatural powers. I don’t really know, but I would suspect that most people using them. This is partly because the reputation they have, and strong opposition to the cards just increases this reputation.
You ask whether people understand that they are ”trading with the occult”. I do not understand what you are asking. It appears that you so mot believe in the supernatural. If you did, I would get the impression that you think anything occult is really bad (i.e. from the devil) and god-fearing people should avoid it. But if one does not believe in the supernatural, then ”occult” is just a word.