Instead of #"moles"#, I could use #"dozens"# as my metric. How many hydrogen atoms are in #4# #1/2# #"dozen"# methane molecules? Well clearly each methane molecule contributes #4# hydrogen atoms; therefore there are #4xx4.5xx12=216# #"hydrogen atoms"#.
In a #"mole"#, #"a Chemist's dozen"#, there are #6.022xx10^23# individual particles; #N_A=6.022xx10^23*mol^-1#.
So in #4.5*mol# of #CH_4#, there are #4.5*molxxN_Axx4# #"hydrogen atoms"# #=# #18xxN_A# #"hydrogen atoms"#.