Max limit of bill items reached

Hi. We are in the process of changing our process to be more compliant and efficient, by utilising bills/bill_items to match against payments. However, when adding multiple items to an existing bill via the API, we received this error:

[message] => bill_items Maximum line item limit reached. A maximum of 40 line items can be entered onto one bill.

Is that a limit of 40 items in one request, or a maximum of 40 items against a single bill?

That seems a rather low limit (and a rather arbitrary number to pick)

Thanks