<% if(Convert.ToInt32(Eval(\"NbrOrders\"))>=Config.MAX_ENQUIRY_SALES) { %> ... <% } else { %> ... <% }
if/else blocks work in ASP .NET as you expect them to. The following works just fine.
<% if(DateTime.Now.Second % 2 == 0) { %> Even <% } else { %> Odd <% } %>
Perhaps the conditional logic in your example is throwing an exception?