1. They evolved from previous languages. Languages evolve out of their predecessors, and branch off from their predecessors. Like how Spanish, Italian, French, etc. evolved out of Latin.
2. Linguists don't even begin to know this. Waaay too complex. There's a theory about Proto-Human (a language some linguists think existed a really long time ago and was the origin of all languages), but it's just a theory, there's not much evidence one way or the other. It's on Wikipedia if you want to read more about it.
3. No, those languages didn't come from English. Again, languages evolve out of their predecessors. Think of like how a family tree works. English and German have a common ancestor (a dead language, not in existence anymore), so they'd be something like cousins. English and French also have a common ancestor, but the ancestor is a lot more distant, so they'd be like more a third cousin twice removed or something by comparison. That's the easiest way of thinking of language evolution.
4. Estimated 6,000. Guesses vary a lot though (it's a hard thing to estimate...). I don't know what you mean by "legit." The estimations include any modern language spoken natively by some population (i.e. it wouldn't include dead languages).