Currently I have six (2) lamp lights on a 15 amp circuit. I plan to adding 3 more of the same style light. They have electronic ballast rated at .49 amps and 32 watt T8 bulbs. .49 x 9 is less than 4.5 amps or 32 times 18 is 576 watts. Am I missing the obvious or am I well with in the capacity of the circuit?

100 watts pretty much = 1 amp.

You are fine.

Add more.

Use the ballast current rating not the lamp wattage. If you consider this a continuous load you can put 24 ballasts on the circuit.

15 amp/125%/.49 amp= 24.48 or 24

I agree with this statement. The wattage of the bulbs does not count