Well, you can have as many junction boxes you want, technically, because the code limits the terminals, not the path to the terminal. Ad I recall, a circuit should have a typical load of 80% of the breaker max, so:
A 12 ga/20 Amp line can run .8 x 2400 = 1920w
A 14 ga/15 Amp line can run .8 x 1800 = 1440w
Then it depends on what's on it. There are rules/guidelines for appliances, including that you need a dedicated line for some.
You're supposed to use 180w (1.5a) for each standard outlet, giving you a max of 10 on a 12 ga line if all you are doing is running branches to random general outlets. I forget how you are supposed to count an overhead light, and technically you're supposed to count an outlet that may have certain things (like an air conditioner) differently.
But to your question, it's not based on junction boxes or number of branches per se, but practically speaking I have a hard time seeing you with many more than 10 if you're limited to 10 outlets.