How to fix a track number?

Self anwer :) 

Hello, huh... ok... sometimes, it's just good to describe a problem and post it in a forum to figure out a work-around. ;P one solution is to insert x normal (not folder) bus tracks at the x first positions in the mixer lane (so their numbers are always 1 to x) and rout the signal from the folder tracks and all other tracks (no matter where they are) to them. As long as you don't move the first x bus tracks across the mixer lane the numbers stay fixed and you can map them to the DAW potis. However. The dealing with folder tracks directly would be much more elegant and convenient.

FYI: Reaper has "universal" tracks now. You can route however you please. That means any track can be used as an aux bus with no routing restrictions (or restrictions on moving/positioning in the board). This may help simplify setup. You can still use the dedicated aux bus 'folder' tracks if you wish.