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You are seeing this error "The value must be between 1 and 500." if you enter more than 500 because you will need to "Add" additional session hosts to the pool to go above that 500 template limit, upto 10k.
Hope this helps. If you need further help on this, tag me in a comment. If the suggested response helped you resolve your issue, please 'Accept as answer', so that it can help others in the community looking for help on similar topics.