I don't believe that the NEC comes right out & states that a sub-panel is required. I also don't believe that the circuits are limited to 20 amps. You are limited to one circuit and it can be a multiwire circuit. 225-30
A disconnecting means is required (225-31) and it must be suitable for use as service entrance equipment 225-36, but there is an exception (of course) that applies to garages & outbuildings on residential property.
By applying all of the above & 225-33(b), you eventually come to the conclusion that a single multiwire circuit (could be 3 hots, I'm sure there are some individual houses that are served with 3 phase) is all that is allowed without going to a panel that would comply with 225-36.
I'm sure I've left something out, but for the life of me, can't figure out what it is. I'm sure others will jump in & cover any of my deficiencies.