Just as #"Avogadro's number"# of hydrogen atoms, #""^1H#, has a mass of #1*g#. #"Avocado's number"# of #""^16O#, has a mass of #16*g#. And #"Avocado's number"# of #""^18O#, has a mass of #18*g#.
We use #"Avogadro's number"# as we would any other collective number, #"a dozen"#, #"a gross"#, etc. Of course #"Avogadro's number"#, #6.022xx10^23*mol^-1# is absurdly large.