Pan Card Contain Alphanewmeric characters and of length 10.
You can use Regular Expression like this
<asp:TextBox ID="txtPanCard" runat="server" />
<asp:RegularExpressionValidator ErrorMessage="Please enter correct pan card number"
ControlToValidate="txtPanCard" runat="server" ValidationExpression="^[a-zA-Z0-9]{10}$" />