If I understand you correctly
you are saying that your ISP is offering a maximum speed of 20Mbps? You will also probably note the words 'up to' in there somewhere, so there is no guarantee you will get the maximum speed. It all depends on your phone line quality, how far you are from the exchange, among other factors. If you want to test your download speed, there are many online speed testers, some being more reliable than others, but you could try
this one.