If multiple screenshots are taken on the same second (like when spamming the screenshot button) only one screenshot is saved for that second. However in older minecraft versions, I remember that all of the screenshots would be saved, by adding a number at the end for subsequent ones so they would have a different filename and not overwrite initial ones.
MC-158359
Screenshots on the same second overwrite previous screenshots
Resolved
Cannot Reproduce
1
I have reviewed this comment.
I haven't tried it myself yet, but I think what this person means is the naming system is bugged.
When a screen shot is created, the name is based on the time the screen shot was taken, down to the last second. When a screen shot is taken in the same second, it has the same name, and because the computer can't have two files with the same name, the first one is deleted to make room for the second one.
You can easily fix this by adding in a system to add a "-1", "-2", "-3" to any screen shots taken in the same second.