The id3v1 spec only has space for a two-digit track number. (IIRC, theoreticaly, you could put any two characters in, so you could count in hex, base-36, or whatever if you need more space, up to a theoretical max of 254**2, with any two chars, other then \0, which probably has special meaning. I don't remember id3 that well off the top of my head.)