How did avogadro get his number?

Yes

It's the number of molecules in a mole of that substance.Eg. 6.02*1023 molecules of Oxygen = 1 mole of Oxygen

atoms in 12 g of c-12

Avogadro number refers to the number of molecules in 12 gram of C-12. Number of molecules clearly implies that Avogadro no. is natural number.

The Number Of Atoms (or molecules) In A Mole of a Substance. Equal to 6.02214179 × 1023 per mole.