Ask people how many different vowels there are in this set of words: trap, bath, palm, lot, cloth, thought. Most English speakers will make out two-four vowels from those 6 words. But they won't agree on which words share the same vowels.

There are other phonological differences between English dialects, but for the most part you can notate them as sounds that merely some dialects don't distinguish (e.g., nonrhotic dialects dropping the `r' sound).