The manta dive doesn’t feel particularly crowded. I did it with Big Island Divers and recommend them - your dive guides are marine biologists and tell you about what’s going on in the ocean, not just what critters you’re seeing.
You go out on a normal dive boat, so you’ll be familiar with the number of divers on the boat. There are obviously several other dive boats moored at the site, and I think there are a couple of ‘torches’ at the site. It’s dark, so you don’t see a lot of people. Each dive op has a specific colored light they put on your tank so they can tell who’s who. They also give you a light for navigating and to shine at the Mantas. When you get to the site, you sit/kneel/grab on the bottom around a ‘torch’ which is a bright light shooting straight up. It’s kind of like a camp fire. There might be 2 dive groups per torch, so your group on one side and another group on the opposite side. I believe there’s a couple of torches but I can’t remember. All I know is there were a quite a few boats and a few snorkel groups, and I never felt like other people were taking anything away from the dive.
Note that it’s not a typical dive at all - you go to one spot on the ground and stay put. Also note that Hawaii has rules about having DM’s on every dive, and I think there’s a ratio they need to keep. Dive ops aren’t going to pay too many DMs so that keeps the total number per boat relatively low.
It was one of the coolest things I’ve done, diving or not. But I got particularly lucky and had a manta twirling directly over me for several minutes. I must have smelled good that day!