for "_i" from 0 to 19 do { _unit addItemToBackpack "20Rnd_762x51_Mag" };
This will perform the code in the brackets {} 20 times and give _unit 20 magazines in the backpack.
"from 0 to 19" means that the game counts 0, 1, 2, 3... until 19 and including 19. Since it's starting on 0, which adds another digit, the code will run 20 times.
if you want, and it would be easier to read, you can also write:
for "_i" from 1 to 20 do {};
More info here: https://community.bistudio.com/wiki/for