Exporting dartboards as png- duplicating board #

Hello

I am on a MAC book pro, OS Catalina, Version 10.15.5,  working in Adobe Illustrator 24.2.1 and I have 16 artboards. They are all variations of the same image so I copied artboards and swapped out images. 

When I go to export as png. it does not export all the artboards as there are duplicate board #'s. I have 2 #10, no 9, 2 #15 and no 14.  How can I fix this? Why is it doing this? Thank you