The quick answer is "many". Every Email client (and I am especially looking at you, Microsoft Outlook) handles HTML emails slightly differently, and so whilst an email might look wonderful on Gmail or Apple mail, it could be an unreadable bundle of code or nasty fonts and colors on Outlook or AOL mail. Images can be blocked, gifs won't play consistently, too many images compared to the amount of text will get your email blocked as spam. The answer is to keep it simple and look for graceful degradation so that the email looks good across the board, but looks even better on some email inboxes