Well..you got a lot of answers that assume what you should want to know but none that quite answer what you asked. Maybe you're just curious?
Two basic slot types for add-in cards.: PCI-E, the newest, and simple old fashion PCI.
The oldest and rapidly being phased out is PCI but some modern motherboards include one or two because people have legacy add-in cards that must use PCI and are costly to replace. These add-in cards are usually very special purpose in nature, like data and video capture or input/output to control industrial process equipment, so it's highly unlikely you'd have a need for it.
The newest...PCI-E...comes in 'flavors' of how wide the the data path to them is. 1 lane, 4 lane, 8 lane and 16 lanes wide. Just like a superhighway, more lanes passes more data but also demands more real estate. They are bigger connectors and the traces running to them take up more space. Also like a superhighway: there's always a compromise since it's costly to use up that real estate for a bunch of lanes you don't really need. So multiple x16 slots will appear on the more expensive boards.
But as the others indicated, you're not likely to be needing multiple x16 slots anyway.