If you send the MaximumItemCountin your listing request, then the number specified for MaximumItemCount will determine the number of items a buyer can bid/buy from you in a 10 day period.
For example you set MaximumItemCount to 25, then any buyer who is currently winning or has bought the 25 of your items in the last 10 days will be blocked with the following message when he tries to purchase/bid on your item which has the MaximumItemCount requirement set.
A couple of things to note -
1. All global My eBay Buyer Requirements are overridden by the BuyerRequirementDetails container. So if you have already specified your buyer requirements in My eBay and you do not want to override any of those, do not send the BuyerRequirementDetails container in your listing/relist calls.
2. My eBay Buyer Requirements will be applied only to listings that were created after the buyer requirements were set in My eBay.
More about buyer requirements