Wintermoor Commander's toughness is equal to the number of Knights you control.
Whenever Wintermoor Commander attacks, another target Knight you control gains indestructible until end of turn. (Damage and effects that say "destroy" don't destroy it.)
The ability that defines Wintermoor Commander’s toughness works in all zones, not just the battlefield.
As long as Wintermoor Commander is on the battlefield, its ability will count itself, so it’ll be at least 2/1.
Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to Wintermoor Commander may become lethal if Knights you control leave the battlefield during that turn.