There are 12 12-letter words that start with 'mag'.
| magisteriums | magistracies | magistrature | 
| magnetically | magnetizable | magnetograph | 
| magnetometer | magnetometry | magnetopause | 
| magnifically | magnificence | magniloquent | 
Note: These 12 12-letter words that start with 'mag' are found from the USA & Canada version of the tournament Scrabble word list.
Also see words start with magn, magi, maga, magd, mage, magm, magg, magu, magl, mago, magp, mags.
4-letter words start with 'mag' (3)