When one says “universe” they denote the entirety of things stuff and possibilities. The maximum. The final boundary. The issue with multiverse theories is they simony replace an entire whole with sub- types high somehow are meant to surpass the the original concept of entirety. They don’t resolve questions like “what is beyond the boundary of the universe” they simply replace it with an extension to which the exact same question can be posited - “what is beyond the boundaries of multiverses?”

Another thing to note is that if energy cannot be created or destroyed it must be finite. It can’t become more or less. Then we must accept that if the “universe” denotes the entirety of finite energy how can there be “multiverses”?

If the law of conservation of energy rings true then their cannot be “infinity” in a quantitative sense. But there can be in a qualitative sense. As energy is the ability to change and if it cannot be destroyed then the capacity for change must be infinite in time. But not in space.