We're ~30 that I know of.
By asking lots and lots of questions, to be honest.
I started with having other people ask them questions for me (when we had no communication or co-consciousness what so ever), then went over to asking them things in documents etc.
Now we let everyone answer things like
* what makes you happy (like, write a list of everything that makes you happy in life, little things, big things, things you like to do etc) - many have answered this
* who you are (this is usually written by those who know the person best of a collective effort. This is just a short "this is who we are, when we were made, how old we are, our name, what our jobs used to be, what our job is now etc.) - we write this together now mostly, and most are on there.
* what we look like (extremely detailed) - many, if not most, have answered this
* inner world (very very detailed. we've let everyone who's wanted to write about their part of the inner world, how it works, extremely detailed what it looks like etc) - most have answered this
* totem animals (this is private so I won't go into it more than this)
It gives is a good idea of who they are and gives us a chance to get to know them. Of course we also let people draw or express themselves through songs etc. Our SO talks to everyone and tries to get to know them. Also, we have M and Leo inside who gets to know most everyone. Lin gets to know all the kids as she watches over them. So asking those we do know well what they know about the others have helped a lot too. Frankie seems to know an extreme amount about the inside and outside world in regards to who's who, who lives where, what has happened and when - but we usually forget to ask her because she's so little

We're starting to learn though

It can be very overwhelming, I know. I'm currently fighting denying some of my newly uncovered alters (I know I have DID, it's just the existence of these "new ones" I'm struggling with). Best of luck