Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Office365 Business Family of plans are a special discounted plan for smaller companies, and thus are limited up to max 300 users per SKU.
For the additional users, you can subscribe to the enteprise plans.
Assuming you are using Office365 Business -> meaning MS-Office on a subscription basis...
The similar plan that does not have a 300 user restriction is Office365 Pro Plus.
So assuming you have 305 staff now, you can mix and match to have -
300 Office365 Business
5 Office365 Pro Plus (yes it cost more, but its just 5 users anyway)
Other "matching" plans are -
Office365 Business Essentials -> Office365 E1
Office365 Business Premium -> Office365 E3
If you can clarify what exact plans you have, perhaps there may be other ways to mix and match.
Cheers