How many cultures still do this?
Where are they located?
I mean, growing your own food, using streams for water and doing without electricity.
What if we ALL did this?
Not enough land? Too many people to make it workable?
In response to your last two questions, ultimately we all do live off the land (our food comes from somewhere, after all) so theoretically it would be possible. Using streams for water would be highly problematic, though–they’d either be drained dry or get so polluted as to be very unhealthful to drink from. There’s also the problem of sanitation, which I don’t want to think about.
Another caveat: I don’t think it would be advisable to grow all your OWN food. People who do that almost invariably end up malnourished because there are a limited number of things you can grow in a given area (especially since, if everybody did this, we’d each end up with a tiny plot). You’d definitely want to engage in some kind of trade.