Feel free to correct any errors!
mass of an average paintball: 3.2 g = 0.0032 kg
300 fps = 91.44 m/s
68 cu inches = 0.0011143 cu meters
4500 psi = 31026407.8 pascals
atmospheric pressure at sea level: 101325 pA
kinetic energy of a single paintball at full velocity:
E = 0.5mv^2
= 0.5 * 0.0032 kg * 91.44^2 = 13.3780 J
stored energy in a 68/4500 tank:
E = pV
= (31026407.8 pA - 101325 pA) * 0.0011143 m^3 = 34459.820 J
number of paintballs fired from the energy in one tank:
34459.820 J / 13.3780 J = 2575.857 paintballs
In conclusion: a marker with a bolt of zero grams, zero friction, zero internal volume (including regulators, hoses, and chambers) firing at 300 fps, at sea level, with an outdoor temperature of 0 degrees celsius or 32 degrees fahrenheit, and firing each ball with enough of a delay to allow the tank pressure to reach equilibrium in between every shot (approx. 5-10 seconds), you can get up to 2,575.857 shots!
note: The 2,576th shot will be a break.