I was talking about the current British Commonwealth, during Queen Victoria Britain was obviously an Empire.

I don't think personal unions count because despite having same head of state the countries remain separate and act each in their own interest so effectively they don't have a single ruling power.

That's why for example The British Commonwealth isn't an empire despite the king of the UK being head of state in all those countries.

I guess it makes sense to say all peoples in an empire should be under the common title though, but I think it can be tricky to define because sometimes you have states that are under a title in name, but practically independent and vice versa.

https://www.britannica.com/place/Mesopotamia-historical-region-Asia/Sumer-and-Akkad-from-2350-to-2000-bce

It depends what you mean by "emperor", if you go by historic definition of a ruler of several peoples, or you go by the "Roman" meaning of emperor, which meant ruler of the Roman Empire.

To fit the first definition you have to satisfy it's requirements, just calling yourself like that won't change anything.

To fit the second option you need to be the legitimate ruler of the Roman empire, and the "legitimate" is the tricky part of the question.

Pretty similar, often "conquering" some city state just meant they pay you tribute.