Use the upsell feature instead of a new subscription. It creates a quotation linked to the existing subscription, priced for the remaining part of the current period.

• Open the active subscription from Subscriptions and click the Upsell button.
• In the upsell quotation, add the five extra licences; Odoo calculates a prorated price up to the next invoice date.
• Send the quotation to the client for online signature or payment, or confirm it internally.
• Once confirmed, the extra lines are merged into the parent subscription, so the next renewal invoice includes all users at the full recurring price.
• Close the duplicate subscription you created with a suitable close reason so it does not affect your churn figures.

The client keeps a single renewal date, and your recurring revenue report reflects the expansion correctly.