I tried uploading various batches of files: 500, 1000, 5000.
While all browsers handle fairly well 500 and 1000 files, 5000 proves to be challenging. It's still possible, but not recommended.
And finally, as mentioned in my previous reply: 50000 are not to be transmitted across the wire; if possible use FTP or even better zip the files and transmit them.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed