You need precisely as many as is necessary to tell your story. Unless you count Wilson, Castaway had one. The same can be said for I Am Legend starring Will Smith. On the flip-side, Leo Tolstoy’s War and Peace has 559 named characters. What it boils down to is the intent of your novel. In my first book, Rebirth, there are only a handful of characters who find themselves at the mercy of an artificial intelligence somewhere in the midst of space and time. Conversely, my second novel, The Drift has an entire town of characters plus others from another civilization seeking to oppress them for their own gain. There…