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.