Legally you could run a single #12 all the way to where you start splitting them out. The problem is the splice in the raceway. If there is adequate space, you can splice in a conduit body.
This is really a gray area since all grounding conductors only count as one.
You are also going to have some serious derating problems with that many conductors in a single raceway so we are not even talking about #12s. All of the wires, including the ground, have to start getting bigger when you go over three 3 phase circuits in a pipe. (9 current carrying THHN)
In the 10-20 wire range you will need #10.
At a certain point more raceways ends up being cheaper.
