On standard maps, you can manage with 3-5 heroes, depending on the size, but in tournaments on RMG maps, you almost always buy 6-8 heroes, but due to the turn limit (8 minutes), you usually don't lose pace and level up 1-2 heroes... 3 in very rare cases...
The only time you might get lucky with experience columns is when playing as a Necromancer, and I once leveled up almost all 8 heroes.
Level 29, 16, 14, 11, 9, 7, 7, 4... there was a lot of excitement, as I managed to do it within 1 game month.
Native heroes, specialists in troops, are always bought, even if there's nothing else to do... is it worth mentioning the usefulness of such heroes as Ilaina, Ingvar, Hafiz, Dugald, Carly, Ossir, etc. + engineers.