I'm looking for an extension that allows to a selected user to add max 100 items(as exemple.)
Do you mean that you want to limit order quantity for each product or each category that specific customers have to meet
If so, I can suggest you two following Magento extensions:
- Limit Order Quantity per Category: allows you to set up minimum and maximum order quantity for all products belonging to a category that certain customer groups have to meet.
- Limit Order Quantity per Products: allows setting minimum and maximum order quantity for a product that certain customer groups have to meet
Both above extensions also customize order quantity limitation for each customer group