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.
![KRobert_0-1618400553786.png KRobert_0-1618400553786.png](https://community.meraki.com/t5/image/serverpage/image-id/17529i6AA5A6546CDE013F/image-size/medium?v=v2&px=400)
https://documentation.meraki.com/MX/Deployment_Guides/MX_Warm_Spare_-_High_Availability_Pair
Hope this helps
CMNO, CCNA R+S