Question

what does the number next to the isotopes signify

Answer 1

there are 2 numbers
Atomic Number
Number of protons
Z
Mass Number
Protons + Neutrons
Whole number
A

Answer 2

I will assume you refer to the one above, that it is the Mass Number, usually the atomic number is not written because is the same for all the isotopes of the same element and correlate with the symbol

Answer 3

thank you @Cuanchi