Not legal in Standard, Pauper, Alchemy, Standard Brawl, Pauper Commander, Premodern, Pre-EDH, Old School
Notes & Rules Information
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.
(2017-02-09)
You can't target the same creature twice to have one recipient get two +1/+1 counters.
(2017-02-09)
Rishkar can be a target of its own triggered ability.