Ok so that 100 Watts is the RMS that your amp sends to the speakers. It's going to draw a lot more actual power than that. As you alluded to, yes it does depend on how loud you have it turned up, too.
If you look on the back - usually near where the power cord goes into the amp, you'll see the more realistic numbers. The Fender Rumble 100 is a 100W amp, but if you look on the back you'll see it's 310W.
So you'd take that and divide by your voltage (310 / 120) and you'd get about 2.5 amps. So you could in theory run about 6 of these on a 15Amp circuit breaker, but good luck lol I bet you start flipping breakers around 4-5 of them.