What as the maximum dosage of topamax (topiramate) usually prescribed to treat bipolar disorder? I am currently taking a total of 100mg a day.

FDA approved maximum. Dose needs to be adjusted to discover optimal individual patient dose. I always find it misguided to use the fda approved dose range as gospel in clinical practice. Your 100mg is below maximum that has been studied by drug company but it does not mean it is helpful or safe to go higher. You need to work closely with your doctor to figure out the right dose for you.
Topamax (topiramate) Fda recommended maximum daily dose of up to 400mg.