If you do not plan on replacing a deactivated user, we recommend removing that user's seat so your subscription is updated with the correct seat count.
When deactivating a user, you will see a summary of all items owned by that user, as well as a reminder to reassign their data if you would like to. There is also a checkbox under Update Billing, which will remove that user's seat from your subscription immediately if selected.
If you would like to remove this seat at a later time, you will need to go to the Billing overview. In your billing overview, you will see your user count, as well as your current subscription cost and your current plan. Under the cost of your current plan, you also have the option to Remove unused seats.
You can read more about what happens to your billing when you deactivate a user in Pipedrive in this article.