Theoretically the % of people needed to reach herd immunity is a function of how infectious the virus is. They use a number R to represent for each person who gets infected how many people they go on to infect. This changes over time as different measures come into place like lockdowns, masks, and social distancing. The basic reproductive number R0 (R-naught) is how infectious the virus is initially without any of those measures in place. The formula for the % of people needed to reach herd immunity is R0-1/R0. So if R0 is 4 then you need 3/4 of the population. If R0 is 3 then you need 2/3.