If that number is above 1, the growth is logistic (exponential in early stages), if it is lower but still above 1, it is exponential but with a longer doubling time ("flattening the curve"), and if it is below 1, cases will decrease.
Washing hands does not protects you 100% so why bother?
Masks are excellent for stopping transmission for infectious patients however - because it catches particulates.
Current (and to be fair, there remains some debate but it seems it is really just the precautionary principle) evidence suggests that spread is by droplet. So a mask doesn’t help stop you receiving it. Definitely helps you stop transmitting it; definitely helps healthcare workers who are exposed to high titre loads, and will be working sick unlike the rest of the population,
But the average person can reduce both their transmission and infection by being vigilant with hand washing, masks if sneezing/coughing/isolating, not touching face/mouth/eyes
Leave the masks to the healthcare workers and the infected
If it were a social norm to wear a mask during an outbreak, and 50% of the population did, it would reduce the spread of the virus greatly. Far from bloody useless. Americans are locked into individualistic thinking: "How will this benefit me?". Also consider asymptomatic transmission.