You should have one wrinkle at the corner of the mouth.
Just wanted to say that this is a guideline, not a rule. My gelding has canines super far back, so he needs his bridle a little bit "tighter", and I know a bunch of horses that don't have any wrinkles at all, but if you check inside their mouths their bits are perfectly adjusted. Depends on the horse's individual mouth conformation.
My gelding has a small mouth and a really fat tongue with fleshy lips [to the point where his tongue pokes out the side when he has a bit in his mouth!], so a thin-ish double jointed snaffle works well for him. A mullen mouth [unjointed] snaffle is the next best thing for a horse with his mouth confo but for some reason he's a snot in one... his pelham is mullen mouth though because a] you're not allowed jointed ones here at the level I compete, and b] he hates ports. He's ok with a mullen mouth shanked bit.
My filly, I haven't had a really good look at, but her mouth seems pretty normal. She'll be in a double jointed snaffle just because I personally prefer them, unless she has some complaint against them in which case I'll use whatever she's happiest in. I'll mouth her in the single jointed full cheek I have just because it offers her a little more support with lateral bit aids. Her bit sits "high" in her mouth because she has really short lips.
Mum's filly used to be mine. I mouthed her with a single jointed full cheek snaffle, then moved her to a double jointed eggbutt snaffle once she'd figured it out. She's very soft in the mouth so she's happiest in a really soft bit. Her bit goes by the one wrinkle rule.
Mum's new gelding is presently in a single jointed full cheek snaffle, but we're going to play, because he's a bit funny about the bit and we want to see if changing it to something else makes any difference. I've offered her my thick double jointed loose ring snaffle [Monty hates it - too thick for his mouth, and he doesn't like loose rings - so I don't use it], and if it fits Ben and he's happy in it then I'll give it to her. We haven't played with bit placement with him yet - we pick him up probably Wednesday - but he has really short lips too so there's a good chance his bit will sit "high" too.