Calcium carbonate is CaCO3.
So there is calcium, symbol is Ca and an atomic number of 20.
Carbon: symbol is C and an atomic number of 6.
Oxygen: symbol is O and an atomic number of 8.
In one molecule of CaCO3, there is one atom of Ca, one atom of C, and three atoms of O as shown by the subscript next to O in the formula. There are no other subscripts to indicate more than one atom of the other elements, so there is only one atom of the other two elements