* Christopher Havel laserhawk64@gmail.com [170526 21:47]:
Quick note... the Pro Micro is a new revision of the older Arduino Micro and has fewer IO pins, IIRC. It may not have enough pins for these purposes...
Yes the naming is weird, you need to watch out for the ones actually carrying an Atmega32U4. As for the number of pins this depends on the size of keyboard you want to make. The arduino is fine for 60% Keyboards. For bigger boards you'd either need a GPIO expander or another controller board. Code for both is available, most people go with the Teensy++. Also as a personal preference I'd take mini usb plugs over micro plugs any day. The latter aren't soldered *through* the board anymore but sit on pads and the physics of that plug makes it likely to damage the plug while fiddling with the cable.
OF COURSE Chinese eBay factories/sellers jumped on the new one. You can still buy the older ones if you fiddle with keywords well enough (or sort through fifteen million pages of crap) but be prepared to pay a wee bit more than you used to be able to get away with... ~$10 instead of about half that IIRC.
The pro micros suitable for said keyboards are around 3 USD...