There is no specific control that covers what you likely experience.
However, what you experience is hard to understand, with the term “number of outputs” not referring to anything clear-cut about API model behavior.
The only think I can think you might be talking about is previously-seen issues where a structured output JSON is not immediately followed by a token to end the response creation, but instead, the model continues writing a second JSON object sometimes.
Otherwise, it could just be prompting technique vs a model not following along.
You can start with reducing top_p from its default of 1.00 to 0.10 and see if more “best choices” in token production get you more of the expected response style.