What is the maximumbreakersiveyou can legally put in residential wiring, not counting a/c unit

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Somewhat ambiguous. The breaker should not exceed the main. Most boxes limit the breakers to 100 A.

In a special case, you could have 400 A service and in that particular box split into two 200 Amp breakers. This box is special and typically mounted outdoors. 400A service is also extremely rare.