Avogadro's number is just a constant and without units. It is alike to a dozen or a gross, only much bigger. 1 mole means that the number of particles existing is 6.022×10²³.
If 1 virus has a mass of 9.0×10^-12 mg, then 10 viruses would have a mass of (9.0 × 10^-12) × 10 = 9.0 × 10^ -11 mg. Likewise, a mole of viruses will have a mass of (9.0 × 10^-12) (6.022×10²³) = 5,419,800,000,000 mg = 5.4 × 10^6 kg, correct to 2 significant figures.