American Express does not include any specific American Express Blue Business Plus credit limit information in the card's terms.
For the most part, American Express Blue Business Plus credit limits depend on each applicant's overall creditworthiness. Those with the highest income and best credit scores will generally be offered the highest starting credit limits.
That said, the American Express Blue Business Plus Card credit limit that you start with isn't necessarily your credit line forever. You can ask to get a credit limit increase by calling the American Express customer service number on the back of your card, but it's best not to ask more than once a year. American Express will also evaluate your account periodically to determine whether you are eligible for a credit limit increase. Paying on time and keeping your credit utilization low are key for getting unsolicited credit line increases with American Express.