Buying reservations for App Service Plan

Hi,

I have a question regarding the purchase of reservations for the App Service Plan. Does it work similarly to VMs? Meaning, if I have a premium V3 App Service Plan that can scale the number of VMs up and down, but I know that the average number of VMs on that specific App Service Plan is 2-3. Do I then purchase 2-3 reservations of premium V3, or does one reservation cover all instances?