How many? Read this article http://arronla.com/2010/08/android-revenue-advanced-task-manager/
Admob is a viable revenue stream. At one point, it generated more revenue for me than my paid application. My free version was never in the highlighted list but it still accumulated a lot of users. I have around 700,000 downloads, daily requests of 300,000, and daily clicks of 3500. Most of the clicks came in from North America, Europe, and Asia. Translation will probably help in those locations.
50,000$ from direct pay(android market) and 29,000$ from adMob (google adsense for mobiles), good money.
