Carbon has 2 naturally occurring isotopes, of which 12C is the most abundant at 98.93 %. Averaged over those abundances the atomic weight is 12.010736 Da.
A mass spectrum does not show that average: it shows one peak per isotope, at the exact mass below, in the ratio of the abundances. That pattern is what identifies the element in a spectrum.
The periodic table, read every way a chemistry course asks for. This tool needs JavaScript; these are the pages it offers: