Mike, Thanks for posting this question.
Firstly, apologies for any frustration experience with this issue.
Just adding to AndreasBaumgarten (thank you!), as mentioned in this documentation, 800 is the # Resource limitation in the template.
-*-800 resources (including copy count)* (screenshot below)
yes, you can export an entire resource group ( as you have done already) or specific resources within that resource group (for reusing the template, as per your needs).
See - Choose the right export option
As I understand (error message), you have more than 800 Resources' in the Resource Group. If it’s feasible, you may consider restructuring the template to use Nested templates:
Link templates for deployment - Azure Resource Manager | Microsoft Learn
Based on your requirement, to deploy complex solutions, you can break your ARM template into many related templates, and then deploy them together through a main template.
Looks like Andreas’s answer was converted into comment by a moderator, since you’d mentioned it was not helpful. Converting an answer to a comment