Can I just test with one force plate?

Yes.

You can only test with one force plate if you would like.

If you choose to only test with one force plate as opposed to two, you will need to use the right plate. The right plate is the force plate with the on/off button. You will connect to this force plate the same way as you would with two. 

If you are performing a bilateral jump on only one force plate it is important to remember that you will no longer get differences between left and right limbs. 

One scenario in which you may only choose to use one force plate is when conducting the ASH (Athlete Shoulder Test).