I chose that number because people who are infected but remain asymptomatic through their infection seem to cause 20 times fewer infections according to a couple of contact tracing studies I saw. It seems pretty clear that the amount of fluid you have to get from an infected person, droplet or aerosol, has to be inversely proportional to their viral load if you're to get infected. Current guesses of an infectious dose I've seen are 100 to 1000 virions via the aerosol route. We can see that different people reach peak viral loads that are orders of magnitude apart which helps make sense of why this disease has such a high k, driven by superspreaders whereas half of people fail to infect anyone else even without protective measures.