Controlling multiple electromagnets

I am working on a project that requires turning on and off multiple electromagnets. I do not need to adjust the current and I believe the current level i need is 500 ma per electromagnet. I am very new to the entire process, but trying learn a lot. I was wondering how I would be able to connect my arduino uno to several electromagnets and control each individually. Can I use more magnets than available pins (multiplexing)? I am learning how to program but I need some advice and guidance before I start coding. I need to turn each magnet on for about 10 seconds and off for 2-3 seconds. I am trying to figure out how many electromagnets I can possibly attach to a single board (more the better). I am sure this is a very novice question but I need some help. Thanks in advance.

will these electromagnets ever be firing at the same time? also, do realise you will not be able to power these directly off the arduino.

Yes, the electromagnets will all need to be turned on at the same time and turned off individually. I understand that a transistor will be necessary to amplify the current. Any suggestions?

People drive huge numbers of LEDs from Arduinos using shift registers, so I don't think you need be concerned about running out of pins. You'd need to provide a driver circuit for each solenoid - basically, a transistor controlled by the Arduino to connect each solenoid to its external power source, and a diode to absorb the back EMF when you turn the solenoid off. Not tricky in electronic terms but if you're new to that sort of thing you'll have some learning to do.

You're looking for a shift register (provides more outputs) and some type of transistor (amplifies an output) to connect to each of those output channels. This post provides a few examples -- notably the post that shows discrete transistors connected to each output as darlington array chips won't be able to handle the 500ma current you need.

It's certainly a bit of a wiring mess for a beginner to deal with. Logos Electromechanical has a nice 4x4 driver shield that you should consider.

I do not need to adjust the current and I believe the current level i need is 500 ma per electromagnet.

That quickly adds up to a lot of current, have you got a power supply that will give you that much? What voltage do your magnets need to get this current.
You would be best controlling each magnet with a logic level FET.
Like shown here:-
http://arduino.cc/playground/Learning/SolenoidTutorial