The # symbol means different things depending on where in the world you are. It’s generally only in North America that it can mean “number” – as in #5 – or “pound” – as in “dial the pound symbol” or “5#” to mean 5 pounds. In the UK, it can’t mean pounds because they have their own symbol for pounds – £.
So where did “hash” come from?
Outside of North America, the # symbol is generally known as “hash”, and the telephone key is called the “hash key”. Hence we have the word “hashtag” for a phrase starting with the # symbol. Oddly enough, hashtags were popularized by Twitter users, though Twitter was founded in North America (San Francisco, CA, United States) and most of the initial users were from the United States.