I want to control 20 (NEMA17) stepper motors with single arduino mega board but gpio pins are not sufficient what is the solution. Is there any chance to extend gpio pins.
What's the problem?
20 motors would require 40 pins (step and direction for each motor). A Mega has many more than 40 pins.
A much more likely limitation would be the ability of a 16MHz Arduino to generate step pulses fast enough for 20 motors if they are all working at the same time.