Since it was several days ago, I'd just let it ride and see what happens. If it eventually stalls for whatever reason, then consider pitching more. Use your hydrometer to tell you if it's still chugging along, not airlock/blow-off activity.
You did under-pitch, so there could be some mild off-flavors, but it will most likely still finish (even if slowler than ideal) and you will still have beer. Either way, this is a great learning experience in regards to watching pitch count.