You have a pricing model where you're monetising the number of seats/users/licenses that your customers opt to purchase. Let's walk through how you can set up a couple of scenarios you might have for a typical seat based pricing plan.
To begin with, let's configure a license add on. To do that, head on to the billable items tab and create a new add on:
Scenario 1 - Customers pick the number of seats they want to buy during checkout/signup
In this case, during sign up, the customer can choose the number of seats they want to start with and then upgrade at any time during the billing cycle.
Attach the license add on to a price plan by heading over to the price plans tab and adding a new price plan.
For the above config, every user would be charged $29 per month on a pro-rata basis. You can also add volume and tiered based pricing for user licenses as well.
Scenario 2 - Customers pay a subscription fee in which some number of seats are included
This config would require an additional block to the pricing plan to cover the subscription fee. To do that, create a fixed fee add on through the billable items menu.
For this config, 5 licenses are included within the $99 subscription plan and every additional user would be charged $29 per month.
These seat based plans can be clubbed with other forms of pricing such as usage based fees, feature credits etc too.