If the greatest power among creatures you control is 0 or less at that time, no mana is added to your pool. It doesn’t use the stack and can’t be responded to. Selvala’s last ability is a mana ability.Note that effects that reduced its power before it left the battlefield will apply.

If the new creature isn’t on the battlefield as the first ability resolves, use its power when it left the battlefield to determine whether its controller may draw a card.If another creature has the same or higher power than the new creature’s power, no one may draw a card. Selvala, Heart of the Wilds - (cEDH) Commander / EDH Aggro Combo Competitive Elves Infinite Combo Mono-Green Tap/Untap dawsonj. The new creature’s power is compared to the power of each other creature on the battlefield as the first ability resolves.: Add X mana in any combination of colors, where X is the greatest power among creatures you control. Whenever another creature enters the battlefield, its controller may draw a card if its power is greater than each other creature's power. TYPE Legendary Creature - Elf Scout ABILITY

Selvala heart of the wilds green white