Can I run 8 4 bulb fluorescent fixtures only on one breaker?
I am assuming the fixture you are referring to is something like a 2X4 layin or surface mount fixture with 4' fluorescent lamps. We used to use a rule of thumb of 200W per fixture. So 200*4 = 1600W or 13 1/3A if you voltage is 120V. These fixtures also need to be calculated as continuous duty or a circuit can only be loaded to 80%. A 15A circuit can only handle 12A worth of this type of load which means a 15A circuit can't handle your load. You can use a 20A circuit which would handle 16A of this type of load. That is what I would recommend.

If you still want to investigate using a 15A circuit, we would need more information on your fixture such as lamp and ballast type.