Be patient. It'll firm up. 4 days is typical but it could take a couple days longer too. If in a week its still not firming up then you may want to look at repitching. Did you use fresh yeast?
There is no real answer. There's too many variables. If you have a tight co2 system (i.e. no leaks) and carbonating and serving on the tank you'll probably be able to push around 10 kegs give or take a couple kegs. They last quite awhile.