Hey!!!!!
Traffic is the key to earn from AdSense. More traffic you get more you will earn. Also Ad Placement and keywords matters too.
Having your own sites would be better than blogs if you thinking long term. because you can place ads where ever you wants.
Its making too many blogs would be useless if you don't have traffic for them.
As far as I know Blogger is the only free blog site which let you use AdSense.
Well its hard to say how much content you need. Again it depends on Ad Placement , Quality of traffic and your content
