I've been trying to make a map for a group of friends to play with. Ideally it would be 8 players against the computer, so at first I tried just placing p12 units on a map and then setting all players to enemy for p12, but that didn't work. I tried using other players in Starforge but it ended up just crashing my BW whenever I decided to test it. I know that each "player" after 8 has an effect but I have no idea what each of them are supposed to do other than p12. Is it even possible to have 8 human controlled players and enemy computer units at the same time?

it depends which sort of map you want to build. For some special cases, your map can work with 8 humans and an enemy.

But you want to have a unit above p8 attacking the players?
That is impossible.

(but you could make player 9-12 to runners in a defense map).

Players 1-12 are safe, however P9-12 are passive. You won't get them to attack. As Ahli said you can use them as temporary players to give units to or as runners, etc.

But I've heard that it's possible to find extended players (P13+) that CAN attack. This is however unconfirmed.
Also extended players, as you said, may crash your map or have other side effects.
It would be great if you invested your time to test and find a useful extended player. Please post your results.
Oh and you can just use ScmDraft for that. Just type the number of the player in the respective box.
If you're not willing to spend that much time I'm afraid you map will be for 7 human players only...

Is there a reasonable limit to the extended players after 12? I need to know if there is a limit before I even begin testing them since 99% of them seem to just make my BW crash.

The owner of a unit is stored as 1 byte, so the highest it'll go is 255.

I think I made the claim that players 13+ can attack (indeed they can), but I dont know if they will attack the players.

Yes, but Player 9-11 can also attack if given the right provocation, it's just too random to actually be used in a map.

And using players past 12 is out of the question, since triggers cannot effect them in any way. You couldn't give any more units to them, all you can do is preplace them, after that you can't do anything else to them.

I'm afraid there's really no way to get extended players to fight back, you might have to change your map idea a bit to work within the limits of StarCraft.

A while back (maybe a year or two ago), I was screwing around with extended units.

Although I'm not fairly certain if they still work, P54 and P56 get blue circles when selected, and will attack each other. They were vultures too.

Yeah, there's some odd extended ones, however, as I said, it won't work since you can't do anything trigger-wise with them.