I haven't worked with the RadTabStrip much so I don't know how to set that problematically; however, I've done something similar by displaying the item count elsewhere on the page. Perhaps this will help you achieve what you desire.
First, you'll should add a HiddenField into the page which will stored the number of items in the RadGrid.
<asp:HiddenField ID="HiddenField1" runat="server" />
Next, in the code behind, stick the number of items int he RadGrid into the HiddenField.
protected void Page_LoadComplete(object sender, EventArgs e)
{
HiddenField1.Value = RadGrid.Items.Count.ToString();
{
Finally, on PageLoad, grab the value of the HiddenField and append it to the tab text.
$(document).ready(function () {
var totalItems = $("#HiddenField1").val();
$("#OpenTradesTab").Text("Open Trades (" + totalItems + ")")
});
Hope this helps.