1. Worldwide the Superhuman population is in the dozens of millions, millions more including supernaturals. Stateside I'd say there are around 30,000 or more metahumans. The population slowly grows with the children of metahumans and those who develop powers spontaneously. Superhumans have changed every aspect of regular society, and some have adjusted to them while others still reel. 

2. The origins of the Awakening are still a mystery but it does tie into magic somehow as it seems to have affected the rates of those who developed magical talents. 

3. Yes, new metahumans Awaken every day from any number of things. Unless it's from a supernatural origin, most superpowers come from the Awakening. Scientific empowerment is possible but its much rarer and has been limited to cybernetic, chemical, and biological enhancements that are still very much in their infancy. This is to avoid all the contrived lab accident origins in comics.