15-Amp & 20-Amp Breakers
The load on the breaker is determined by what you plug into the outlets, not by how many outlets there are on the circuit. If you have a 15 amp circuit, and you plug in a 15 amp toaster, your circuit is effectively maxed out. As to overloading the breaker to keep it safe, if the breaker is working correctly, it is safe, since it will blow if you overload it. Code does not specify a maximum number of outlets on one circuit, although realistically the more outlets you have on a single circuit, the greater the probability you will plug in too many devices and trip the breaker.