The amount of organisms in a given space is called the

Population, community, or environment

Explanation:

It depends on if you are asking the amount of the same species, the place they live in,  or if you are asking different organisms

If you are talking about the same species than the answer is population

If you are talking about different species interacting it would be community

If you are talking about the place in which they live, it would be environment :)

Hope this helps, I'm just a little confused on the question :)