Counting by percent. If there were 100 people, this would reduce it to 75:
Revelation 6:8
8. And I saw, and behold, a pale green horse, and the name of the one sitting on it was Death; and Hades followed after him. And authority was given to them to kill over the fourth of the earth with sword, and with famine, and with death, and by the wild beasts of the earth.
Of the 75 that are left, this would reduce it to 50. Hence, 50% of the population killed.
That is a lot of graves.
Rev_9:18 By these three was the third part of men killed, by the fire, and by the smoke, and by the brimstone, which issued out of their mouths.
That is a lot of graves and a lot of burial.
Usually in cases of mass death, the dead are simply piled up and burned or long trenches are dug for mass graves.