any suggestions on how to limit items which each customer purchased?
for example:- 18 March : Mr.A bought 2 pieces of ItemX.
20 March : Mr.A bought 1 pieces of ItemX.
22 March : Mr.A bought another 4 pieces of ItemX.
therefore, total ItemX that Mr.A bought is 7 pieces.
are there any mods, addons, whatever to limit Mr.A to purchase only 5 ItemX accumulatively?
Nothing commercially that I'm aware of. You would need to hook into the add_to_cart and then sum all of Mr. A's items he has ordered and then dsiallow the add_to_cart if the condition was exceeded.. That's a lot of overhead to scan all of Mr. A's orders, especially if Mr. A is NOT a registered user for every add_to_cart.
How do you intend to manage the max number of each product that can be purchased?
All this is possible with some custom development.