You don't really need more than four digits total. There is almost zero chance you have even a thousand pieces in your library in any of those categories unless you literally list every concert band piece together. But it's up to you.

My categories are, with prefixes listed:

UIL List Music (CU)
General Concert Band (CB)
Marches (both concert and small) (CM)
Marching Band (MB)
Jazz Band and Mariachi (JM)
Sightreading (SR)
Holiday (XM)

I don't use sub-categories. Each category gets its own cabinet/section.

The prefixes go before the number so that I know where a piece goes in the library. I've seen some people simply number a piece and slap it at the end, which is fine - but my way works for me.

With UIL, I *always* list them as four digits - but the first digit is the UIL grade. A grade 5 could be 5001, for example. Helps sort them in the cabinets too. Everything else is just labeled based upon category. Since there are only about 150 pieces in the Grade 1 list, you could concievably list them with three digits, since it's unlikely you own the entire list, but that's up to you.

Also, fun for your chosen envelope size: if you cut them in half with a paper cutter, each "half" envelope is precisely the right size to file your marching band music without wasting a huge envelope for small music.

Couple tips:

As you go through, DISCARD photocopies. If you have a piece that you're missing more than 5-10 "parts" for and you don't know anyone who has it, you'll need to rebuy the piece anyway, so it's up to you whether or not you should pitch the whole thing.

If it's a song that legitimately hasn't been popular in 30 years or more, THROW IT OUT. No one in your band wants to play "Sounds of the Carpenters" in 2022, no matter how lovely the music was.

Keep marches. You can find some gems you've never discovered. Discard pop tunes. Really look at your holiday music to see if you'll EVER want to program it again - holiday music can be VERY VERY badly scored.