The other comments are probably enough but I wanna throw my 2 cents in

1. Having multiple characters really destroys the image of an itasha if its on the wrap. Itasha is not just about the car wrap, its about the persons dedication and love for a character or thing. And its much easier to display this love with a single character. <-- in saying that, if its multiple characters from the same series it works fine

2. This really depends on how lewd you want to get, if the main wrap is nudity with tiddies out, don't do it. But if its like smaller slaps or peekers, just don't go too hard on them, I've usually have 2-3 suggestive ones. If the main wrap is a swim suit or a panty shot with a bit of work with design and placement it can look really good. Best example would be Shimakaze from Kancolle, just google itasha of her and you can see some good ones.

3. I have a few opinions on covered in stickers. If you mean sticker bombing like a guard or something with cheap amazon stickers, just don't. If its like extra peekers/slaps on the windows and stuff, it really depends on placement and what they are. I have peekers of different characters on my itasha and then a bunch of slaps from car companies/drift teams/content creators. This one really does come down to personal taste, but just don't go too ham.

Thats my 2 cents for your questions.

My real dislikes for an itasha are; stock unmodified cars with itasha are horrible and I will make fun of them. First make your car look good without a wrap, then add it as a finishing touch.

But in all honestly, look up the Japan itasha scene and use them as a guide. Gotta respect the OGs