Can I use cartridge bearings on my wheel hub?
To use cartridge bearings, the hubs must have been designed to use them (absurd kludges aside).

Your hubs will almost surely be classic cup-and-cone hubs, and those will need replacement bearings. 

However, do note that the proper number of bearings for a hub is not necessarily a number that completely 'fills' the cup bearing race. You might not be missing any. 9 is typically common, with a little bit of space where it looks as though you could fit a 10th or 11th. Check with your LBS for how many you'll need for your particular hub.