Most (maybe all?) of the trolls have an associated number, some more obvious than others. The one that I like to bring up is Eridan's being 311, which is apparently because of the emote it makes when the "11" part is in romanumerals: 3XI ... It's also a date in March...

Actually, I decided to check a couple wiki pages on the ones I didn't know off the top of my head and it seems it is all of them.

Aradia - 0

Tavros - 1

Sollux - 2

Karkat - 612

Nepeta - 33

Kanaya - 6

Terezi - 413

Vriska - 8

Equius - 100

Gamzee - 420

Eridan - 311

Feferi - 380

As for the 612 origin: most people attribute this to Karkat's birthday, ie the start of Hivebent being 6/12/10, but it was also actually the day Bard Quest began, on 6/12/07. I believe this was the first original content made for MSPA, since Jailbreak (although posted the week prior) was just a mirror from a forum. So it's kind of MSPA's birthday too.

(https://web.archive.org/web/20111222021620/http://www.mspaintadventures.com/?viewlog=2)