Can you use digital pins as PWM output?
Only specific Digital Pins marked with ~PWM can generate a PWM signal. In the case of arduino Uno the pins 3,5,6,9,10,11 are all hardware PWM driven, and are marked on the Uno with a ~ or something.
Others can be driven with software PWM(though I wouldn't recommend it). As the Arduino Uno uses the ATMEGA328P, the number of pins is same with other boards using same processor. 

On the other hand if you want to run 16pins for PWM, its best if you use Arduino MEGA 2560 because its processor supports more than 6 Hardware pins for PWM. Also you get the advantage of a higher memory storage for your code