Dive tank = 1 good days play......
How many fills? You'll get the first few at the pressure of the tank, then it will gradually diminish until you get to a point where you think why bother. Their are a lot of factors to consider, amount of shots per game, efficiency of your marker, size of your bottle, etc.....
If your at a standard day where you would be shooting say no more than 3000 paint, then your dive tank should keep you in air for the day.
However if you got mates with dive tanks, do as mentioned above, using the bulk fill and top up routuine and you should experience better fills for longer.
Hope useful.
Oh, and my recomendation is to get your dive bottle filled up to the max pressure of your pball air system only, i.e. 3000psi. All it takes is one little slip and you will pop a burst disk ruining your day balling, or worse pop your tank and ruin your life. Potentially their is more energy in that little bottle than you'd ever truly appreciate.