(I'm curious and Googled for this but get a lot of unrelated stuff.)
Reasoning: (Number of infected, N) / (viral load)×(average human size in milliliters)) × (mass of the virus) × number of time viruses replicate in humans.
N = 2-10 million, Viral load: 10^4 to 10^7 / mL so maybe 5x10^9 viruses per human at any time. The weigh of the virus is in the order of magnitude 1×10^−18 (100s attograms to femtogram)