The figures I've seen that "looked" reliable were that wearing a mask would reduce the percentage those you infect by about a third.
Wearing it for a healthy person would reduce chance of infection by about 5%.
So, if you're contagious, and would normally have infected 3 people, it would be only 2 if you wore a mask in company. The R naught I see is usually lower than 3 of course, more around 1.2, so wearing a mask would reduce it under 1.