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If you keep, youre not playing to the strengths of your deck

10 Sylvan Library Sylvan Library is one of the best card draw effects that green has access to in the Commander format

Its important to note that just activating the regeneration ability doesnt exactly mean the creature has been regenerated. The oracle text makes it clear that this gets a +1/+1 counter when it regenerates , so it still needs to die in some way for regenerations replacement effect to actually resolve

Stopping your opponents from casting spells

Some deck themes built on creature synergies can also influence how many creatures are best for the build: Typal Youll want as many of the chosen creature types as possible if you decide to run a typal deck