Maximum File Count

Limit the number of files users can upload using MaxFileCount. Once the limit is reached, additional files are rejected.

Maximum 3 Files

<au:AjaxFileUpload ID="Uploader1" runat="server"
    AllowMultiple="true"
    MaxFileCount="3"
    ButtonText="Select Files (max 3)"
    AutoUpload="true"
    ShowProgress="true" />

Try selecting more than 3 files to see the limit enforced.