How Stripped Down Can a Language Get?

Do we need thousands of words — or even verb tenses — to communicate?

James Austin wants to know:

How stripped down could a language be and still function well and not just for communication but to live in? Could you sketch out what that would look like? For example, how much would it need in the way of tense, aspect, and mood; would it need a counterfactual (like “si” in French or “too” in English); how many words would it need — 10,000, 2,000, fewer? more?; how many phonemes would it need; and which subject pronouns? More generally, what features would you strip out of a language or not include in one that you were building?