Does commander damage count from all combat damage dealt over the whole game or just one turn?

Commander damage is tracked cumulatively across the entire game, not just a single turn. A player loses the game the moment they have been dealt 21 or more combat damage by any single commander over the course of the whole game. This is a state-based action checked continuously (CR 704.6f).

Each commander has its own separate damage counter tracked against each opponent. So if Commander A has dealt you 15 damage over several turns and Commander B has dealt you 10 damage, you are not eliminated by commander damage — you need 21 from the same commander (CR 903.10a).

Only combat damage counts toward this total. Damage dealt by a commander through non-combat means (e.g., a burn spell, a triggered ability) does not count as commander damage (CR 903.10a).

If a commander is removed from the battlefield and later returns, its previously dealt combat damage still counts toward the 21-damage threshold. The damage total persists even if the commander changes zones (CR 903.10a).

Example: Your Voltron opponent attacks you with their commander for 7 damage on turn 5, 7 damage on turn 7, and 7 damage on turn 9. On the third hit (total = 21), a state-based action is checked and you lose the game — even though no single attack dealt 21 damage.