2020-11-10 : If Opal Palace's last ability produces two mana (most likely due to Mana Reflection) and you spend them to cast a commander, that commander enters with two counters for each time it's been cast from the command zone this game.

2020-11-10 : If you don't have a commander, Opal Palace's last ability produces no mana.

2020-11-10 : If you have two commanders, the last ability adds one mana of any color in their combined color identities.

2020-11-10 : If your commander is a card that has no colors in its color identity, Opal Palace's last ability produces no mana. It doesn't produce {C}.

2020-11-10 : The "number of times it's been cast from the command zone" includes the most recent time. For example, the first time you cast your commander from the command zone in a game, if you spent mana from Opal Palace's last ability to do so, it will enter the battlefield with a +1/+1 counter.