Both prenda and ropa mean clothing. Although prenda also means jewelry in Venezuela but in general contexts, it seems to be synonymous to ropa. Or isn't it?
Prenda is normally an article of clothing and thereby countable, whereas ropa is clothing in general and not normally countable (unless you're talking about the various types of clothing, in which case it is).
- Me compré ropa ayer. (cuantidad desconocida)
- Me compré dos ropas ayer. (poco común)
- Me compré una prenda ayer. (una pieza)
- Me compré dos prendas ayer. (dos piezas)
This is the general usage in Spain, however, there may very well be some regional differences.