Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Two things to notice:
One, you can add more than 300 users for a single tenant.
Two, however, Microsoft stated that "Organizations may only provision up to 300 seats total across all of Business family of plans (Business Basic, Business Standard, Business Premium)."
So, this is not a supported design: 200 × Business Standard plus 200 × Business Basic = total of 400 Business licenses (exceed 300 cap)
A supported approach would be 200 × Business Standard plus 100 × Business Basic plus 100 × Microsoft 365 E3 (or E5)
Microsoft also recommend organizations with more than 300 users should consider subscribing to Microsoft 365 for enterprise plans.
Ref: Microsoft 365 Business Premium frequently asked questions | Microsoft Learn
I hope this information clarifies your question. If you have any other question, please feel free to ask me.
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