Hi there, I actually had the same questions as you, and after stumbling upon this, I decided to spend an hour or so on google, which wasn't all that fruitful...
What I did find is something I hope you (and others) might find useful:
1. On fuel:
I didn't find anything specific, but there are aviation fuel storage tanks available up to 100.000 litres (Which is about 100 tons if I'm not mistaken...)
source: http://airfuelsystems.com/refuelling-solutions/aviation-fuel-storage/
2.A NATO-squadron usually consists of anywhere between 10-24 aircraft, but smaller or larger numbers are also possible (source: wikipedia). An airbase can house multiple squadrons.
3. On Ammo:
About the only thing I could find on this was this article (interesting read btw..)
http://usmilitary.about.com/library/milinfo/milarticles/blafnavy.htm
but that doesnt't really answer our questions.
4. On civ planes:
The number of civilian airplanes depends on the size of the airfield.
Usually I use the following site for my civ. airtraffic. It has info on almost all airfields used in DCS, and it should give you some idea of the number of aircraft.
http://www.flightstats.com/go/FlightStatus/flightStatusByAirport.do?airportCode=TBS&airportQueryType=1
5. I have no clue.
Whew, that turned out a bit longer then expected. I hope this is in any way useful, eventhough it doesn't really answer our questions all that much...
Perhaps some of the RL miitary people here can provide us with some more info?