I'm actually not sure how many dresses she had! I haven't researched MA in particular. Her enemies were really harsh about her conspicuous consumption, but that meant differently then than it does now. It's smart to consider what would have been typical for European upper aristocracy.
I know that for those who had money but weren't absolutely rolling, clothing was a priority and not cheap. Their fabrics tended to last a long time, particularly silk and wool, and there are many, many gowns still in existence that show extensive re-fitting and re-making through decades. (Silk gowns from the 1740s were being re-made even in the 1840s.) Cotton was a newer, luxury material, that was imported largely from India. By MA's time it wasn't super expensive, but it was still far from a utility fabric.
Una, you might enjoy this article. (There's some language.) http://www.frockflicks.com/the-gross-18th-century/
