How many circuits should I include in a sub-panel for workshop?
This may not be worth the extra expense and work of installing a sub panel. Looking up some prices from the blue hardware store:

6/3 roll 50 ft       $102        12/2 roll 250 ft   $48
subpanel              $60        3 breakers         $20
3 subpanel breakers   $20        boxes and outlets  $25
main panel breaker    $20
12/2 roll 50 ft       $25
boxes and outlets     $25
-------------------------        ----------------------
total                $255        total              $93


I think 3 circuits is appropriate. One for the tool you're actively running, one for dust collection, and a spare if you need to be running two tools at once.

Edit: If you plan on upgrading to tools that will draw more than 20 amps, it may be worth the extra expense of putting in a subpanel. That way you can use shorter runs of 10/2 or 10/3 from the subpanel to the tools, instead of running all the way back to the main panel.