I think theres 1 person who has solod Kara, and maybe a handful of others who double box it. So thats not a ton

Naxx 10 and 25 man isn't too bad, once your groups get some decent gear I can see doing it with 2-3 people. There are lots of movement (Heigan dance, 4H rotations, Sapph iceblock hiding), PBAOE, kiting (Gluth), etc issues that imo make it too difficult to solo imo, but a few people and it will get easier. Even some of the early bosses have technical aspects, like neeing to mind control an add on razuvious while a tank grabs another add and all the DPSers/healers move in and out of LOS. Thats 2-3 precise movements required, simultaneously, and he's one of the first bosses you fight.

Dunno really about the later tiers of raiding though, haven't touched any of them but from what I've seen on the beta realms they do ramp up in difficulty.