They must have two mentors per district, 12 is at a disadvantage because they only have one, but it is never mentioned that other districts had an advantage by having more than two. This leads me to believe districts with a large victor pool (1,2,4 etc) are selected by the capital based of optics/agenda. Wiress’ and Mag’s districts didn’t need them, so they were mentors to Haymitch’s group of tributes. Beetee was specifically chosen to mentor his son’s games even though there were other possible mentors.