The number of instant and/or sorcery cards in your graveyard matters only as you begin to cast Swift Reckoning. Once it's cast, that number doesn't matter and will have no effect on Swift Reckoning resolving.

If an effect allows you to cast Swift Reckoning from your graveyard, you need two other instant and/or sorcery cards in your graveyard to cast it as though it had flash.

Check to see if there are two or more instant and/or sorcery cards in your graveyard as the spell resolves to determine whether the spell mastery ability applies. The spell itself won't count because it's still on the stack as you make this check.