You draw X cards and you lose X life, where X is the mana value of a commander you own on the battlefield or in the command zone.
If you own multiple commanders on the battlefield and/or in the command zone, you choose one of them as Stinging Study resolves to determine the value of X.
If your commander is under the control of another player as Stinging Study resolves, it is still on the battlefield and will still be used to determine the value of X.