answer: 4 oxygen atoms. it's in the name..Na2CrO4
might be wrong but I hope this helped
element symbol atomic mass number of atoms mass percent
Sodium (Na) 45.9795 g/mol 2 28.3871%
Chromium (Cr) 51.9961 g/mol 1 32.1017%
Oxygen (O) 63.9976 g/mol 4 39.5112%