Under the US-NEC the number of residential circuits is determined by
calculation and the load is to be approximately evenly divided between
the circuits. (At 3 watts per square foot a 20A circuit is good for 800
The bottom line for your garage is what you are doing is fine unless you
are going to overload the circuit. If you are going to use the garage
for a shop figure what the maximum load will be at any time.
The NEC has no maximum number of receptacles on a residential circuit.
(Local codes could.)
For other than residential, in general a receptacle (or duplex
receptacle) is to be counted as 1.5A each.
I agree with Bill's approach. (For "continuous" loads (over 3 hours) a
breaker should only be loaded to 80%.)
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