How many fluorescent lights can be can on 1 - 20 amp circuit ?

Though you can normally determine amperage of a device by dividing the wattage by the voltage, this does not produce a correct result because of the power-factor of the device when it is something like a fluorescent light or electric motor.

Unless the amperage is printed on the box or the fixture, you will need to open the fixture, located the ballast (usually a black rectangular box) and read the amps from the label. Another option would be to use an ammeter and actually measure the current.

Once you know the current, you would be able to figure out how many lamps the circuit would stand... but...

The National Electrical Code (NEC, I'm making the assumption you are in north america) requires a circuit to be loaded to No More than 80% capacity if the load could be operational for 3 hours or more, you may fit into this category which means the max load to the circuit is now limitied to 16 amps. Divide 16 by the amperage per fixture to determine the max number of fixtures.

Just guessing, each fixture is perhaps 0.75 amps so you could have no more than 21. Get the idea?

Rich is very close to correct, if not right on. To get an exact number, look on the ballast for the amp rating. Divide that number into 16. That assumes only the fixtures and switches will be on that circuit.

what is the start up current of 12 x 5 foot flourescent light fittings @110v, takeng in to account that the running curent is 0.75 amp