How many lights can be on one breaker?
As the other two posters have indicated, it depends.

For 20 AMP you have 20A*120V or 2400 watts and 15 AMP you have 15A*120V or 1800 watts.  Good Electricians will only use 80% of the circuit and leave a 20% safety margin so you have 1920 and 1440 watts respectively at your disposal per circuit. 

Lets use 1440 as a conservative estimate.

Next, find out what's on the circuit.  To do this, leave the lights on that you want to connect to.  Open your electrical panel and flip off the breakers one by one until the lights turn off.  Now turn off all the breakers except for the one the light are on and see how many lights and outlets are powered by the circuit.  It's then just a matter of adding up what is on the circuit.  Let's say you have 8 50 watt lights on the circuit, that roughly 400 Watts of draw (roughly).  That gives you and extra 1000 watts, more than enough to add a few flourescents.  Adding up the lights are easy.  But the plugs will be a little more difficult, because there is a difference between having your entertainment center (TV, stereo, etc) on the circuit versus an alarm clock radio and just a lamp.  For the current ligths and appliances that are connected, you can plan whether adding another light is feasible.  Some circuits are lightly loaded, others not so much, so your mileage will vary.