Is it true that Avogadro did experiments to determine Avogadro's number?
He didn't. Avogadro's number was simply named after Avogadro.
The Number Of Atoms (or molecules) In A Mole of a Substance. Equal to 6.02214179 × 1023 per mole.
Yes
It's the number of molecules in a mole of that substance.Eg. 6.02*1023 molecules of Oxygen = 1 mole of Oxygen