Hi Rohit do the following way
<div>
<Ajax:ToolkitScriptManager runat="server"></Ajax:ToolkitScriptManager>
<asp:TextBox ID="txtItemCount" runat="server" MaxLength="1"/>
<Ajax:FilteredTextBoxExtender ID="ftItemCount" runat="server" ValidChars="123" TargetControlID="txtItemCount" FilterType="Custom"></Ajax:FilteredTextBoxExtender>
</div>
Thanks and Regards,
Rk_Hirpara