This line $('#<%=TextBox1.ClientID %>').
assign the DatePicker to the input with the ID=TextBox1
and only to this one, so its work as you say.
To add more DatePickers you can add more lines with the rest controls that you wish to use the DatePicker, or set the same css on this control you with to attact the DatePicker and change the code to $('.DateKeeperCss').
where you set the DateKeeperCss style to all Text Editors that you wish to have the DatePicker. For example:
$(document).ready(function () {
$('.DateKeeperCss').datepicker({
showOn: "button",
buttonImage: "../images/calendar.gif",
buttonImageOnly: true
}).regional['fa'];
});
and
<asp:TextBox runat="server" ID="TextBox1" CssClass="DateKeeperCss"></asp:TextBox>
<asp:TextBox runat="server" ID="TextBox2" CssClass="DateKeeperCss"></asp:TextBox>
<asp:TextBox runat="server" ID="TextBox3" CssClass="DateKeeperCss"></asp:TextBox>
<asp:TextBox runat="server" ID="TextBox4" CssClass="DateKeeperCss"></asp:TextBox>