Limit the number of files users can upload using MaxFileCount.
Once the limit is reached, additional files are rejected.
Maximum 3 Files
Drag & drop files here, or paste from clipboard
<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.