What is the empirical formula of N2O5?

You... have it already. You gave the molecular formula, something by which the molecule is actually represented without any reduction of subscript ratios. But...

The empirical formula is the molecular formula with the most reduced subscripts you can get.

##5## is (of course) an odd number, and ##2## is obviously an even number, but clearly you can't halve ##5##.

Therefore, ##"N"_2"O"_5## is already an empirical formula.