if you look at http://www.wingate.com/products/wingate/licensing.php you need to have an Enterprise license to use more than 1 http cache volume. If you don't have an enterprise license, you can create more than 1 volume, but only 1 can be used to store and serve files.
thanks for your reply i am having professional license and i don't want to use more than one volume but i want to know is there any size limitation for cache volume because i created only one volume