Grand Warlord Radha

Haste
Whenever one or more creatures you control attack, add that much mana in any combination of {R} and/or {G}. Until end of turn, you don't lose this mana as steps and phases end.

Dominaria

Dominaria Promos

After Radha’s triggered ability resolves, you can cast spells and activate abilities before blockers are declared.

The amount of mana you’ll add is the number of creatures you attack with. Creatures that are put onto the battlefield attacking before Radha’s triggered ability resolves don’t count, and creatures that attacked but left combat before the triggered ability resolves do count.