you can try this:-
your GridView:
<asp:gridview id="gridRow" runat="server">
</asp:gridview>
your button and textbox:
<input type="button" id="btn" value="Total Row" onclick="rowcount()" />
<input type="text" id="txt" />
your javascript function:
function rowcount() {
var row = document.getElementById('<%= gridRow.ClientID %>').rows.length;
document.getElementById('txt').value = row;
}
Good luck.