I want to restrict no of orders by customer to 99 per month and the total amount limit to $500 per month.


I suggest that you will need to create an extension with an observer that watches for the quote_totals, something like the following: http://www.blog.magepsycho.com/tag/event-observer/

This will need to check against your conditions and probably prompt the user in some way.

Another option would be to do it at the template and/or block level. Checking against the conditions there may be easier to implement, but would not be as future-proof.

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