If you add more users and it increases your subscription tier on an annual plan, you will be charged a pro-rated price for those additional users the day after your tier changes.
You will receive an email in advance of any subscription tier increase so that you know it's coming. If you archive/delete users from your system and it moves you down a subscription tier, you will need to schedule a tier "downgrade" for your next renewal if you'd like to stay in that lower tier moving forward. Otherwise, your plan will renew at the tier that you most recently paid for, either via a renewal or a prorated charge for a tier upgrade.
To schedule a downgrade, follow the steps outlined in this support article: