300-seat cap
Microsoft's limit of 300 licensed users across the entire Microsoft 365 Business plan family within one tenant.
Business Basic, Business Standard, and Business Premium share a single 300-seat ceiling. It is not 300 of each. Once you need user 301, no further Business licence can be purchased.
The supported answer is a mixed tenant: keep the Business licences you have and put additional users on E3 or E5. Converting everyone to Enterprise is usually the more expensive path and is rarely necessary.
Where this comes up
300-seat cap sits inside our microsoft 365 and cloud work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.