Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Have a look at the following link
https://www.microsoft.com/en-sg/microsoft-365/business/microsoft-365-frequently-asked-questions
Look under Plans & Billings -> (10) Can I mix and match Microsoft365 plans
Our Microsoft 365 for business base plans (charged per user) are designed for organizations with up to 300 users. Organizations with more than 300 users should consider subscribing to Microsoft 365 for enterprise plans. We reserve the right to enforce a tenant limit of 300 provisioned licenses across the family of business plans, in which case we will provide advance notice and further guidance. In the meantime, we are treating customers that have provisioned up to 300 licenses of each individual business plan (Microsoft 365 Business Basic, Business Standard, Business Premium) as compliant with this 300-seat limit. This applies even if they have provisioned more than 300 total licenses across the family of business plans.