Explanation
The two possible causes of this behavior are that the client is using a Professional edition Org which limits the number of records that can be processed using API, and that users are assigned to the Default Maps Permission group that has the Max Query Size defined to 5000. These factors can affect the number of records that users can plot on Desktop using their Account based marker layers. The Professional edition Org has a limit of
5,000 API calls per user license per 24-hour period, which can restrict the amount of data that Salesforce Maps can access and display. The Default Maps Permission group has a default setting of 5,000 for the Max Query Size, which can limit the number of records that users can query and plot on Desktop. These factors are explained in the Salesforce API Request Limits and Allocations document and the Salesforce Maps Permission Groups document.