How to restrict user to create only certain number of document in a collection

How we can restrict user to upload only certain number of document in a collection

Same as any other validation: use an Appwrite Function to do validation before creating/updating

is there is any other option?

except this

Nope. Functions are the way to go for any sort of custom behavior

example if you can provide, for how to restrict via function

The idea is instead of using the create document API, you would force users to execute a function. The function can do validation and only create the document if validation passes

yes you are right, but how do i know that how many documents is created previously or how will i count the document

You'll need a way to query for it using the list documents API