Since there are so many uses for USB, most motherboards are capable of having a ton of them. The board below has 4 on the back of the computer, as well you can plug an additional 2 into each expansion connector on the motherboard. It's labeled number 4 in the picture below. The view below is the same board without the number.
Before buying a PCI card for more USB plugs, check the motherboard to see if it supports more like this one does. The adapter for the motherboard will cost you a lot less.
Currently there are 3 standards of USB, just USB will give transfer speeds of 1.43MB/s. USB 2.0 goes up dramatically to 57MB/s and the new USB 3.0 standard will give another MASSIVE jump up to 572MB/s making this universal bus even more practical for any use you may have for it.