Hi Shweta, this is the standard behavior, and as far as I know there is no customizing to restrict the number of follow on documents that you can create.
As a workaround maybe you can implement some code in badi check (or another badi you choose) in order to check if for that RFx response already exist a follow on document and in case yes trigger an error message to avoid the creation.
hope this help!
Celeste