There are 74 total characters to unlock. Your story mode playthrough will unlock 29 of them, and playing through the bonus level will unlock Sauron and Mouth of Sauron.
43 additional characters will be found wandering around various areas of Middle Earth for purchase. Use your map to make it easier to find them.
The last two characters you will unlock are Blacksmith and Lurtz (Newborn). Upon forging all Mithril items, you will unlock Blacksmith, and upon achieving 100% completion, Lurtz (Newborn) will be unlocked.