Quoted from a study determining how much masks reduce the mortality rate of COVID-19: "In countries with cultural norms or government policies supporting public mask-wearing, per-capita coronavirus mortality increased on average by just 15.8% each week, as compared with 62.1% each week in remaining countries.
Another study found that US states who mandated the use of face masks in public had a greater decline in the infection rates of COVID-19 by up to 2 percentage points per day. Another found that the people in Hong Kong, about which 96.6% of them wear masks and is ranked the 3rd most densely populated areas in the world, have a significantly lower incidence rate of COVID-19 (about 129 cases per one million people) than other countries like France or the U.S, who had 1151.6 cases and 1102.8 cases per one million people respectively.
Note that an important factor in increasing the effectiveness of masks is to wear them properly and making sure that they fit! COVER YOUR NOSE WHEN YOU'RE WEARING A MASK!