Hash / Pound Sign

Special characters on Mac keyboards are different:

Character	Mac Keyboard (US)	Mac Keyboard (UK/ISO)
#	Shift + 3	Option + 3 (UK keyboards!)
~	Shift + backtick	Option + N (some layouts)
\	Regular backslash key	Option + Shift + 7 (some layouts)
@	Shift + 2	Option + 2 (UK) or Shift + 2 (US)
€	Option + Shift + 2	Option + 2

Tip: Press and hold Option to see a keyboard overlay of special characters. Press Option + Shift for even more.