Change your code  as below mentioned 

1.

function isNumeric(e) {
 var c = getKeyCode(e);
 if (c != 8 && (c < 48 || c > 57)) {
 return false;
  }
 return true;
 }    

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

or for checking numeric values simply use below mentioned code in your design

2.

 <asp:TextBox ID="txt_numeric" runat="server" MaxLength="2"  onkeyup="this.value=this.value.replace(/[^0-9]/,'');"  Width="50px"></asp:TextBox>

Hope it helps you!!