Ignore the "set minimum to 2". That's weird advice that either does nothing or directly prevents dwarves from training properly. Might have been an override for a bug once? Not sure.
you want the minimum to be at least 2, so that dwarves scheduled for training have someone to spar with
and you want the minimum to be less than 10, so that you can simply leave all your squads set to training/active all year (no micromanaging of training schedules), but still have your military dwarves working or socialising as needed
if your population is big enough to support a standing military of full-time soldiers with no other labours, the default "minimum 10" is probably fine, but few forts can spare the dwarfpower in the early years -- but you do wanna get them training (at least a little) asap
i usually start at "minimum 2 training" when i first assign a melee squad, increase it to 4-6 once it has 10 members, and then just leave it there
crossbow squads stay at "minimum 2" on training, even when they're full, cause by the time i have 10 marksdwarves i usually have ramparts for them to patrol as well: 2 training, 4-6 on patrol, 2-4 free to socialise/craft/etc
[edit: patrolling ramparts is useful for spotting werecreatures and ambushes before they reach your gates, and also ends up being a bunch of free training cause they'll take potshots at any birds/etc that pass by]