This has happened to me a couple of times on 2 different devices. I believe it happens when I go through a long list of emails very fast and keep tapping the delete icon. I noticed one popped up again and I deleted it again and then it shows -1 when I have no mail and 0 when I have one new one, for example. I worked on this for days and the only way I could get rid of it was to delete that particular mail folder (it was for my local internet provider's email service) and then reinstalled it and it was fine.