Can I use cartridge bearings on my wheel hub? I have a vintage road bike, at the moment there are loose metal bearings in the hubs. Some of the bearings are missing. Can I buy cartridge bearings or do I need to buy loose bearings?

Thanks
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.