What is Script Manager in Ajax+ASP.net

ScriptManager control is the parent control that needs to be there on every page wherever we are trying to use ASP.NET AJAX controls. ScriptManager control manages client script for AJAX enabled ASP.NET pages. This control enables client script to use the type system extensions and support features for partial page rendering, webservice calls etc.

Can we use Multiple Script Manager on same web-page

No. It is not possible to use multiple ScriptManager control in a web page. In fact, any such requirement never comes in because a single ScriptManager control is enough to handle the objects of a web page.