how many bubbles per minute?

Get a CO2 indicator, and aim for 30-40ppm.

The bubble rate is adjusted to inject no more than 20ppm/CO2 at lights out. Then see what your CO2/ppm is at lights on and adjust the bubble rate up or down from there. You don't want to go over 30 ppm and if you inject over night the ppm will increase because the plants aren't using the gas in the dark.
You can find your ppm by testing for kH and pH and typing them into the calculator that can be downloaded here:
http://www.csd.net/~cgadd/aqua/art_plant_aquacalc.htm

Len