Tracknumber tag

What tag should I use if I want to numerate my tracks from 2CD albums ignoring track tag but as they are in the list?
For example, not "01 - Beatles - Birthday" but "18 - Beatles - Birthday"

There is no rule for that.

ID3 standard is using the two tag fields discnumber and track.
Many people ignore discnumber and give use the tracknumbers with strarting from 101 for cd1 and starting from 201 for cd2. (that's what I prefer).
But you can also just number them as if they were one cd, with 18 for the first track of cd2. If you use discnumber in addition you can still see if the track is on the first or second cd.
Or you let the original tracknumbers as they are and invent an additional custom tag field, e.g. TRACK TEMP.
Or you use the filename for an additional numbering system.
The best thing will be using a way which your audio player can display. Very few players can display custom fields. Discnumber and Track are displayed by almost all players.