One mole of anything is 6.022xx10^23 of anything, including molecules.
1.5color(red)cancel(color(black)("mol HCl"))xx(6.022xx10^23"molecules HCl")/(1color(red)cancel(color(black)("mol HCl")))=9.0xx10^23"molecules HCl"
Every "HCl" molecule contains two atoms, H and Cl.
9.0xx10^23color(red)cancel(color(black)("molecules HCl"))xx(2"atoms")/(1color(red)cancel(color(black)("molecule HCl")))=1.8xx10^24" atoms" rounded to two significant figures