I know that if I gain control of my opponent's commander, it won't lose the status of commander. How is the commander damage calculated in this case? Can I do the 21 commander damage with the stolen commander to win the game? Does the damage add to the damage of my own commander?
Yes, you can make a player lose by dealing 21 total damage to them with their own commander. (If it keeps getting stolen by yourself or others, a cumulative total 21 damage from their own commander will make them lose.)
The rule, from mtgcommander.net's Rules — Play section, is:
- If a player has been dealt 21 points of combat damage by a particular Commander during the game, that player loses a game.
That is any one specific commander card, including their own. Commander damage from different commander cards doesn't count together: if you've successfully stolen every commander on the board and dealt 20 damage with each of them to one opponent, they still haven't lost to commander damage.
Also, commander damage never makes anyone win directly, it only makes players lose. You'll only win if them losing made you win, e.g. they were your only opponent.
1If you've dealt 20 damage with 4 or 5 different commanders, most decks will have lost by then, not due to commander damage, but due to being dealt 80-100 damage.– AndrewDec 30, 2017 at 15:00