You can use Copmpare validator for this
<asp:textbox id="txtCOB" maxlength="3" runat="server" xmlns:asp="#unknown">
size="2"></asp:textbox>
<asp:comparevalidator runat="server" xmlns:asp="#unknown">
ControlToValidate="txtCOB"
EnableClientScript="True"
id="valCoB"
ErrorMessage="Please enter a number"
Operator="DataTypeCheck"
Type=Integer></asp:comparevalidator>