Hello!
I'd like to expand arduino mega pwm pins for controlling DC motor. (with H-Bridge)
I have to control over 20 DC motors however I could not find the solution for this.
I found out that there is a TLC 5940 to control LEDs. Can I use it for controlling DC motor too?
OR, Is there any solution that I can solve it...?
No idea about the TLC 5940; too lazy to downlaod the datasheet 
There is Adafruit 16-Channel 12-bit PWM/Servo Shield - I2C interface. To my knowledge you can can use it to drive H-bridges. You still will need to control the direction using other pins or port expanders.
If you only need to control in motors in one direction, MOSFETS are easier than H-bridges.