I played the game for the 1st time last month - it was hosted by a friend and I have no real knowledge of the rules beyond what was explained to me - Hopefully someone with better rule knowledge will weigh in, but in the meantime for what it's worth, in the F&IW scenario we played the Indians had six figure units and most of the 'European' units (British & French regulars & militia) were in eight figure units. I seem to recall Indian units max size was six figures, but there was a bit more flexibility in the European unit sizes.
My rule of thumb for wargames is a player should have at least three maneuver elements and that would hold true for M&T, while double this number of units would not pose a challenge to an experienced player, I would think.