The Arduino's AVR (Atmega328p) microcontroller for example has 3 ports, each of which can have up to 8 pins. PORTB has 6 pins, PORTC has 6 pins, and PORTD has 8 pins. Which is a total of 20 GPIO pins. In other words, a port is a group of pins that can have up to 8 pins and those pins are attached to the same IO control registers (DDRx, PORTx, and PINx), where x can be (B, C, or D).