First, you may wish to consult our Discounts help page here:
http://www.e-junkie.com/ej/help.discounts.htm
Bear in mind that Item Discounts (defined in each product's settings) are entirely separate and unrelated to Cart Discounts (defined in Admin > Cart Discounts). Item Discounts do not have any setting for Max Use Count.
Cart Discounts do not apply to any specific product; they only apply to a whole order for any of your products. In any given buyer's cart order, there can be at most only one Cart Discount applied only one Item Discount code applied per product as well.
Max Use Count is a setting you can define for a specific Cart Discount, so that discount can only be applied that many times before it expires -- this is often used for promotions like "First 100 customers get 15% off", or for creating Gift Coupon codes that can only be redeemed once.
The instruction about "enter multiple codes with the same settings" would be useful for creating a series of single-use Gift Coupon codes that all have the same discount amount settings, so you could sell those codes as Gift Coupons on your site. See more about this approach here:
http://www.e-junkie.com/ej/tips.gift-coupons.htm