If you are using them together as an HA pair, then you only need 1 license. If you are using them separately, then you will need a license per appliance. If you are using them in an HA pair, then like @MarcP said, they have to be the same model. 2xMX84, 2xMX100, 2xMX67W, etc.. In your case, since you are purchasing two MX84s, you'll be okay.
Hope this helps
CMNO, CCNA R+S