Update quotas-limits.md

Author: mrbullwinkle

articles/ai-services/openai/quotas-limits.md

@@ -51,6 +51,7 @@ The following sections provide you with a quick guide to the default quotas and
 | GPT-4o max images per request (# of images in the messages array/conversation history) | 50 |
 | GPT-4 `vision-preview` & GPT-4 `turbo-2024-04-09` default max tokens | 16 <br><br> Increase the `max_tokens` parameter value to avoid truncated responses. GPT-4o max tokens defaults to 4096. |
 | Max number of custom headers in API requests<sup>1</sup> | 10 |
+| Message character limit | 1048576 |
 
 <sup>1</sup> Our current APIs allow up to 10 custom headers, which are passed through the pipeline, and returned. Some customers now exceed this header count resulting in HTTP 431 errors. There's no solution for this error, other than to reduce header volume. **In future API versions we will no longer pass through custom headers**. We recommend customers not depend on custom headers in future system architectures.