The registration comes in the mail as a standard envelope delivered by first class mail. The envelope comes from the Library of Congress so do not throw it away. The second line of the return address says United States Copyright Office so read the return address well if you are expecting them. The eCO system is very good and the registrations come much faster than the website tells you they will. If you have questions the personnel at the USCO are very good and quite friendly. They are however very busy so give them some time to get back to you. You may call them also.
The questions continue even tonight about how many should you register, how often and how many do I have registered. Well, I cannot tell you how many to register. If you shoot professionally or want to do so and send work for editors to look at than all that you submit should be registered prior to sending them. Make sure you mention that fact in your letter for the submissions. I am in the process of registering a lot of images. I use thumbnails as they are quick to upload and you only get an hour for each session. I try to finish mine all at one time in that hour. I am currently starting to register a couple of hundred or more each month. With the missing HR files I need the protection. I currently have about a thousand images registered with a new application being worked. The current rate for electronic registration is $ 35.00 but new rates are expected. The $ 35.00 is for a work which can include a large number of images. If you are doing something other than photographs you may have to do things separately. I did some registrations that had to be mailed and that takes longer.
Just think about it. If you have a few thousand images that are properly registered and someone uses just one it can be worth as much as $ 150,000 plus any DMCA violations which can also be as much as $50,000 each. That makes a good case for taking care of the registrations. That single sheet of paper can be very valuable.
Just remember to register your work. An old saying says it best about trying to be a photographer or anything else. Do not be afraid to fail, be afraid not to try.
I am not an attorney. These are my personal opinions and general information only. These are not legal advice and should not be used as such.