More than you might think. Ale-wife was probably the most common, if a woman brewed ale at home and made more than her family could drink before it would go "bad" she frequently sold it. There were rules and regulations even then, so we know about these women from the records of the fines they had to pay for violating the rules.
Another common "profession" was prostitution - there were no laws against it until the 20th century.
Women were midwives, laundresses, milk maids, wet nurse, spinsters, knitters, tranters (sellers of small items) and (my favorite) leech catchers (walk bare-foot into the marsh, pull up your skirts and walk around in the slimy water - walk out and peel the leeches off your legs) there are others, but these are the most common for lower class women.
Middle-class women worked at the same trade as their husband or fathers - there are many instances where widows continued to run their husband's business after his death. Many middle class women entered nunneries, since payment was generally required by the nunnery, poor women generally didn't talk the vail.
Upper-class women didn't have professions as much as they had "positions" - Lady-in-waiting, Countess, 1st Lady of the bedchamber, 1st Lady of the Queen's stool - that type of thing.