The actual Arduino MEGA board shows PWMs 0 to 13, but pin 0 and 1 are also marked TX and RX and included as part of the communication.
According to the table 13-14 in the Data sheets, this is incorrect. It may be that that TX0/RX0 could be used as a PWM, but it is most likely used as the main TX/RX for communication, thus leaving us with 12 PWM as stated in the data sheet.
So that just leaves us with the one extra pwm to figure out.
EDIT
After laying out the ATMEGA1280 in Eagle, I come up with 14 pwm pins as stated on the arduino page, but 2 more than what the data sheet states - and this is without useing the TXD0/RXD0 as shown on the Arduino MEGA board.