Nidus Mutation Per Stack

[Imaru]

When Nidus was released, his Undying passive cost 10 Mutation Stacks per proc. It was then changed to 15 later on. 

As you probably know, when he gains Mutation, various parts of him open up, but this still maxes out at 10 stacks, not 15, which would be consistent with the former way it worked.