To see how many and which computers you have registered under your account, all you have to do is login to your web account, and click on the profile link on top right corner of the page.  Once you are in the Profile section, you can click on the Devices tab and this will allow you to see the details of the devices you have registered under your account, last time each device was backed up, stored data size of each device, whether it is connected and active or not, and the used space of your account.