Hi Subbu,
Thank you for your interest in Syncfusion Products.
Please refer the below link for samples files which illustrates of getting the TreeView items count value from server side using CallbackMultiplexer and without posting the page.
http://websamples.syncfusion.com/samples/Tools.Web/F77851-TreeView_CallBack.zipPlease let me know if this sample helps you.
Thanks,
Rekha