Custom validation expression creation

Summary: We have 6-7 alternate hierarchies in the Account dimension and the requirement is to build a custom validation which validates that all the alternate hierarchies are having same count of level0 members in the dimension. For Example Primary hierarchy is having 100 level0 members then all the shared hierarchies should also have 100 level0 members if not then validation should give an error while validating the viewpoint. Can someone please guide on this whether this can be achieved via custom validation expression or not?

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