A lot. At least two of everything, for any weather. I also started wearing my biking clothes a long time ago when not riding a bike. Most of my wool tops, hats, socks, gloves, undergarments, etc. work as well as regular clothing. Although, a lot of the stuff I wear when riding is not biking specific: I have some tops and jackets meant for other sports, rugged hiking and work clothes, etc.
Basically, when I buy clothing now I want it to be multipurpose: on the bike and off the bike.