vast: so overwhelmingly great, in strength or in sheer numbers, that it cannot be resisted
This word describes something whose size overwhelms. Depending on the setting that size is either force or number, and often it is both at once. A nation, an army, a swarm of locusts, or a flood is called this when it is simply too big and too strong to stand against.
Because it fuses might and multitude, the same word slides between two shades in English. When the thing in view is a fighting force or a mighty figure, it means powerful and overpowering. When the thing in view is countable, like herds or sins or a crowd, it means abundant, a formidable mass. The Bible often pairs it with the ordinary word for many or great, and in those pairings this word usually carries the extra note of overwhelming force while its partner carries plain quantity.
The root behind it points to being vast and formidable, and that single idea holds the two shades together: whether the greatness is counted or felt as power, it is the greatness of something you cannot push back.