I’m afraid there’s no way to know the exact number, but I can tell you that I have screenshots or pictures of fourteen characters. The ones I played Dead Is Dead are not all documented except in the DiD threads over at the Beth forum. I’m not inclined to go dig through all of those threads!

If you count games on different platforms for the same character the number would be higher. For example Jerric alone has I believe four Oblivion games (all started from scratch in the prison cell) across the Xbox, PS3, and my laptop. He may have had five at one point. But I just counted him as one character.
Also I didn’t count the characters I have made through the character creation process but not really played much. I wouldn’t count them since they were not intended to have a life independent of their out-of-game stories.