It is difficult for me to give numbers when so much depends on a person's lifestyle and budget. The experts who wrote Harper's Bazaar's book, Great Style, did come up with a list that I think is pretty good. Here it is:
4 sweaters (2 work-worthy and 2 for weekends)
7 tees (short and long sleeves, tanks, and turtlenecks)
4 coats and jackets (1 elegant coat, 1 sporty outerwear jacket, 1 classic trench, and 1 day-to-night jacket)
2 tops for every skirt and pant (1 dressed-up and 1 casual style)
4 pants, 4 skirts (2 office and evening appropriate and 2 informal)
1 suit (for every season, that you can wear together or separately)
5 dresses (2 for day, 1 short for cocktails, 1 long for formal, and 1 special knock-'em-dead frock)
4 jeans (1 slim cut for a cleaner look, 1 relaxed fit for laid-back days, 1 ankle length for flats, and 1 longer pair for higher heels)
What do you think of this list? So much depends on what your regular activities and work requirements are. I don't think everyone needs 5 dresses. What if you don't like dresses? Still, I think this list is okay as a starting point to think about what you really need.