a large number: a great quantity of things, a crowd of people, or the whole assembled body of a community
At its core this word simply means fullness of number: a lot of something. What that something is shifts with the context. It can be a great quantity of things (a haul of fish, a pile of sticks, stars beyond counting, a heap of sins), a crowd of people packed into one place, or the whole membership of a group considered as a single body.
That last use is worth watching. In the book of Acts the word often names the gathered community or an official council acting together, not just a random throng. Translating it "crowd" there can mislead, because it points to an organized whole (the believers, or the ruling council) rather than a chance gathering.
So the reader chooses the English word from the setting: a quantity, a crowd, or an assembly. The one idea underneath them all is the same, a great many gathered or counted together.