When Rishkar, Peema Renegade enters the battlefield, put a +1/+1 counter on each of up to two target creatures.
Each creature you control with a counter on it has "{T}: Add {G}."
Each creature you control has Rishkar's mana ability as long as that creature has any kind of counter on it. The effect isn't limited to those with +1/+1 counters.
Rishkar can be a target of its own triggered ability.
You can't target the same creature twice to have one recipient get two +1/+1 counters.