Mac keyboards don’t have a # (‘hash’ for the UK, or ‘pound’ if you’re American) key, which is a bit of a problem when you want to type a # symbol.
The lack of a dedicated key doesn’t mean you can’t type the symbol, though – just type [Alt] + [3] or [⌥] + [3] (depending on your keyboard) and the # symbol will appear.
There’s just one thing to note – you need to press the [3] key on the top row of number keys as the [3] key on the numeric keyboard won’t work.