A web page should not contain more than one ScriptManager control. But the question is how to work with with multiple web pages which are linked to a Master page.

A ScriptManager control should be declared in a Master page and ScriptManagerProxy controls should be declared in all child pages. So that ScriptMangaer services can be included in all child pages.

<asp:ScriptManagerProxy ID="ScriptManagerProxy1" runat="server">
</asp:ScriptManagerProxy>