We don't have a way to display the sort of notification you described, but you can set up a discount that's only good for the first 5 buyers. This help page explains our discount settings in detail:
The Max. Use Count setting determines how many times a discount can be used before it expires, so you would set up your discount with Max. Use Count = 5. That setting will not work with our Buy Now buttons, so you would need to be using E-junkie Cart buttons to sell your product with this type of discount.
You can set up the discount with a Discount Code that buyers would need to enter to claim the discount, and this would display a "discount expired" message to anyone entering the code after its Max. Use Count has been met.
You could just leave the Discount Code setting blank to have the discount automatically applied to the first 5 orders that meet whatever conditions you specify for the discount (Product and Min. Item Qty./Total); however, in this case once the Max. Use Count has been met, the discount would simply stop being applied without showing any message about this to buyers.