The nRF52840 Dongle has 15 GPIOs in addition to the ground, power, and SWD connections along the castellated edges.
The castellated edge holes have a pitch of 0.1 inches (2.54 mm) and a row spacing of 0.6 inches (15.24 mm)1, making it suitable for stripboard connection. Test points for additional nine GPIOs are available on the back side of the circuit board, leaving a total of 24 GPIOs accessible.