Look at a herd, of pretty much anything that comes in a herd. Or a school of fish. The entire life cycle of these animals occurs in the group. That's full social. A fish doesn't need 'alone time.' Neither does a cow. Contrasted with solitary animals, such as bears, or certain species of sharks. These animals come together pretty much only to mate, though they may tolerate adults of their species in 'their space,' depending on species. Humans are somewhere in the middle, depending on your temperament. We need a certain amount of social interaction to stay healthy and happy, but that doesn't mean huge groups. This is where we diverge from biology and get into psychology quite a bit. Biologically speaking, humans are semi-social because reproduction primarily, as well as other important stuff, takes place in private, or relative private, and if you keep a big group of us penned up with food, water, sanitation, but nowhere to be alone, we get crazy also. Both perfect solitude and full herd behavior are anathema to the human animal.

Psychologically speaking we could go for years (And have been) On the actual difference between introversion, extroversion, and related topics. All that we've been able to clearly establish is that some people love periodically (Periodicity also being a variable) being in massive groups with other people, and others prefer occasional interaction with a few people (Again with number and periodicity being variable). All of that falling on a spectrum with extremes and outliers at both ends, but with most people falling under the meat of the bell curve.

A cool idea in this area of study is Dunbar's Number which is basically the idea that you can only have meaningful (different degrees of meaningful here too) relationships with a set number of people. The proposed range is somewhere between 100-250 individuals.