Question

WHY SODIUM AMALGAM OXIDATION STATE IS ZERO?

Answer 1

The oxidation number of Sodium would be +1 as all group 1 metals are always +1. Mercury in Sodium Amalgum even though it is usually +1 would have an oxidation number of -1, as 
Mercury is less electropositive, or less likely to donate electrons than Sodium , it takes on the -1 charge in this case. 

The sum of oxidation numbers in a neutral compound should always add up to zero i.e -1+1 = 0.