@Manisha Rathi The form recognizer free resource has a limit of 5 models per compose and the paid resource has a limit of 100 model per compose. If you are using the free tier you can upgrade your resource pricing tier from free to get the benefit of using upto 100 models per compose. As per the current functionality if you plan to use new trained models as a composed model it will be a new compose model id, an existing compose model id cannot be updated.
If you are using the rest API you can follow the steps mentioned in this thread to create and get the compose model id.