Presumably, there is a step between the NLP and the actual image generation.
Could be function calling, could be some other mechanism. It seems rather trivial to restrict the system at that interface point where the NLP gets translated into image generation instructions and simply cut it off at a limit.
Since there is an interface point where the NLP is translated into some image generation call path, presumably, the response generation can see that it only got 1 image back instead of n. Even a system prompt could be added to normalize the output text portion of the response.
The image generation API returns a URL to the image, which ChatGPT isn't going to fetch and look at. If ChatGPT asked the service to generate 500 images, it seems reasonable to assume that the returned URL would be some sort of zip file or gallery or even single image grid containing those 500 images.
Alternatively, if ChatGPT does think it is suspicous that it only got one URL, it might end up responding with "the system seems to be not working right now, I'm only getting partial results for your query", because it doesn't know that the system is only going to return a single image.
This is getting into speculative territory, so I guess the true answer could also be "OpenAI are amateurs are prompting ChatGPT", but it seems less likely.
Could be function calling, could be some other mechanism. It seems rather trivial to restrict the system at that interface point where the NLP gets translated into image generation instructions and simply cut it off at a limit.