The oxidation number of Na in Na2CrO4 is +1 as it is an alkali metal. The oxidation number of oxygen is usually -2 unless it is in a peroxide, superoxide or OF2. Thus, the oxidation number of Cr in Na2CrO4 can be calculated as follows:

Let x be the oxidation number of Cr. Then,

2(+1) + x + 4(-2) = 0

x -6 = 0

x = +6

Therefore, the oxidation number of Cr in Na2CrO4 is +6.