Non-organized responses off the top of my head:

I'm not sure how I'd define "real" here (I kind of hate the NPC notion some people use). But going with "authenticity," I'm not sure it's a restricted by a number as much as the ability to spend time and energy on friendships. To a degree this is a variation of wide vs deep (#498633), but instead of knowledge, it's friend groups.

I don't think authenticity is in any way a link to loneliness and isolation. If you're spending your time going deep into friendships, that's the antithesis of isolation. If anything, I'd feel that people who go wide, with tons of shallow "friendships" that have nothing behind them, are much more likely to feel lonely.

I do think the number of deep friendships someone can have differs a lot because of a lot of factors (including introversion, but also things like age - how many of us had more deep friendships as kids at least partially because of the time we spent with so many other kids all day? -- and probably some intangibles).

I also can't really imagine trying to quantify my deep friendships. I've often only realized how strong a connection was well after it firmly took root.