You are not required to use a subpanel and there is no minimum amount of circuits. You could legally run 14/2 or 12/2 UF buried 24'' on a 15 or 20A GFCI breaker.

"You are not required to use a subpanel and there is no minimum amount of circuits. "

If you do not have a panel you are limited to a single circuit 9though it could be a multi-wire circuit.

If you have a sub-panel you can have as many (or as few) circuits as you want (though if you only have one or even two it is a bit of a waste since a single multi-wire is allowed an could provide two 120 V circuits).

The panel size should be based on what loads you plan on running.

If the loads are as light as you say a single 20 amp multi-wire circuit is probably still more than you meed, and then no panel is required.

As Brickeyee mentioned, for what you describe a multi-wire branch circuit would be just the ticket. I'd run 12/3 (plus a ground) UF cable. That would give you two 20 amp circuits, which should be plenty of power and would be less expensive than a sub-panel.