I have a boil under my armpit, all the puss is gone but there is a little pin hole im wondering if i must need to buy antibiotics to treat this?

Can not advise: Would need either to see it or a photograph. Probably needs nothing more if was a large superficial pustule or infected ingrown hair follicle. A boild needs to be adequately drained, so it could reform if the opening is only pinhole in size. Would need antibiotics if area aound it is red +/or tender as this could be early cellulitis.

Possibly: Use warm compresses and it my go down without an antibiotic. If it flares up see a doctor for treatment.