Number of atoms are calculated using a number known as "Avogadro's number."
Okay, lets see a breakdown of this. Let's say that you are given an amount of grams of a substance. For this case, lets say that that substance is Carbon (C). And, lets assume that you are given 4.01 g of Carbon, and you are tasked to find the number of atoms in that mass of Carbon. The breakdown would be as follows, with dimensional anaysis:
4.01
g Carbon
(
1
mol Carbon
12.01
g Carbon
)
(
6.022
⋅
10
23
a
t
m
s
C
a
r
b
o
n
1
mol Carbon
)
=
2.01
⋅
10
23
atms Carbon
Basically, I first wrote down the amount in grams, and I used the molar mass of Carbon (which can be found on the periodic table under Carbon) 12.01 g/mol to convert 4.01 g of Carbon to moles of Carbon. Then, I used "Avogadro's Number", or
6.022
⋅
10
23
atoms per mole
to convert the mole amount to atoms of Carbon.
The process should be very similar with other such atoms, just make sure to keep your periodic table and calculator handy.
I hope that helps!