if I select as a condition, "company located in US", and only 1-3 other very broad conditions, I get 0-1 companies...this isn't possible...I have the same result with UK...

The screener selects a subset of the stocks that are in the universe of exchanges you set from the Markets menu in the upper left of the screen. If you are not selecting the US or UK exchanges from the Markets menu, the screener will return 0 results when you attempt to find companies in those markets.