What atom mass is 12 and has 6 electrons?

This is the element Carbon.

Carbon has three isotopes, of which carbon -12 is the most common.

It is composed of : -

C-12 ; 6 protons, 6 neutrons, ( 6+6 = 12) and 6 electrons.

The other isotopes of carbon are#

C-13 ; 6 protons, 7 neutrons, ( 6+7=13) and 6 electrons

C-14 ; 6 protons, 8 neutrons, (6+8=14) and 6 electrons.