It really depends on the truck, some may be a tanker unit capable of pumping anywhere between, 500-5,000 Gal. or More. You might even see a Brush truck unit, that has a simple 2 stroke engine pump, that does not have a tank, but a pump unit, these can pump aprox 250-1500 GPM (Gal Per Min).
But to define a per second bases would be like this.
Lets say for example, a pumper with a water capacity of 2,500, able to pump that amount in 2 mins. This would equal 20.8 gal aprox per SECOND times 60 seconds per min times 3 mins would = 2496 gal total. so, a Rough guesstimation would be about, 20.8 gal per second on this example unit.