How many roommates are allowed in university dorm?

I have only ever played in Britechester-U, but I just took six Sims through uni. I was intending for two of their 'neighbors' to be NPC attendees. But I stuck them all in the very same dorm, Wyvern (which actually states in game that it has ten beds, five more than it shows when first selecting it). Turns out I was in control of all six. Each took a full load, so that I could be done in three Sim weeks. This was one more Sim than I had previously controlled in uni. The only adverse problem was my young Global Superstar fell in love with both the neighbors-turned-dormmates. LOL There were a ton of NPCs in this dorm all of the time. Probably because of Erik's fame. Happily, I controlled none of these.

As for your second question. I do know that teens can live without adults. I'm uncertain if you can have a teen and a child from CAS without an adult. I do know back in 2014 when I was playing Pinstar's Legacy Challenge, both my Founder and later his wife perished on the very same day, leaving their three boys, 2 teens and a child on their own. They managed. All you can do is try, unless someone else comes along with a more definitive answer for you.

Best of luck and I do hope I helped. :)