Try this

 function isNumeric(e) {
var c=e.keyCode? e.keyCode : e.charCode
 if (c !== 8 && (c < 48 || c > 57)) {
 return false;
  }
 return true;
 }   


       <asp:TextBox ID="txt_numeric" runat="server" MaxLength="2"  onkeypress="return  isNumeric(event)" Width="50px"></asp:TextBox>