How much does Lotus Bloom heal?

The heal scales off Leafy's own max HP, so more HP on Leafy means more healing for everyone. Lotus Bloom is Leafy's ultimate, and the 15% is worked out from Leafy's pool, not from each teammate's. Your low-HP damage dealers get the same flat amount as your tank.

The heal lands instantly and reaches the entire team, so you don't need to group up or wait for it to tick. It also gives the whole team a 15% Grass elemental damage boost. In a Grass team you are buffing your damage dealers at the same moment you top them up, which makes it a good ultimate to fire just before a burst window.

Leafy's other healing

Lotus Bloom is Leafy's big heal, but it isn't the only one. Leafy's turf circle gives a small team heal. Vitality Blooms heal only Grass-type teammates who are standing inside the area, so a non-Grass teammate gets nothing from them. That is one reason Leafy, a Grass boss Aniimo built around regeneration and healing, belongs on Grass teams, and why it is a top-priority teammate for Irisalis.

How it compares with Melobloom's heal

If you are deciding on the best healing Aniimo for a Grass team, Melobloom is the other support to weigh up. Its rose mark blooms after a target takes three critical hits and heals all nearby allies for 5% of Melobloom's HP. That is a smaller heal than Lotus Bloom, and it depends on landing crits. Melobloom covers different ground: it cleanses debuffs and negative status effects from the whole team, which Leafy's kit doesn't do.