Each click of the Download button accounts for one credit 

Understanding Download Credits
Downloads work on a 24hour clock. Every 24 hours after the last download and NOT a set time, i.e. midnight.

As an example, and imagine you have a 5 per day credit allowance and if you downloaded something similar to the following;

file1.zip at 10:30...download again 24 hours later at 10:30
file2.zip at 11:45...download again 24 hours later at 11:45
file3.zip at 14:25...download again 24 hours later at 14:25
file4.zip at 15:40...download again 24 hours later at 15:40
file5.zip at 19:49...download again 24 hours later at 19:49

and so on...

Therefore, your credits will vary depending on when you download, but it is still correct as of the 24-hour clock.