Error Message:
Collection size 1,006 exceeds maximum size of 1,000

There is a published salesforce limitation of 1000 queries in VisualForce and Salesforce fails to loop through all templates by template ID to find the one to use Document generation. Visualforce is used to process the document generation and is launched when you click a button so if you see this error message when trying to generate document request, then your org hit this limitation.
-In this error message, user has 1,0006 email templates which is 6 over the 1,000 limit for email templates. So this user needs to delete at least 7 templates from their org to bring them to have 999 email templates in your org or less. Once your total email templates are 999 or less, you should be able to continue with document generation.