A total of 494 Electoral Votes are in play for write in candidates.
Currently, 43 States allow Write In Ballots for President of the United States.
States not allowing write in ballots include; Arkansas, Hawaii, Louisiana, Mississippi, Nevada, Oklahoma, South Dakota
Most States require a candidate to register, however; Vermont, Wyoming, Oregon, New Hampshire, New Jersey, Iowa, Delaware, and Alabama do not require registration.
Thirty-five states require that a write-in candidate must submit some form of affidavit and, sometimes, a filing fee at least one month before the election. In North Carolina, these candidates must circulate a petition. Then their names are posted on a list at the polling place, though not on the official ballot. All other write-in votes are tossed. ~Bloomberg