What microcontrollers are suitable for a keypad
Based on you comments, it sounds like you want:


An 8-bit AVR, because that is what you are used to
A USB device port, so you can communicate with the computer
A cheap chip, not so much to save £1, but so it doesn't feel like overkill


You can find that by going to this page on the Microchip website (Microchip now own Atmel, and make the AVRs): https://www.microchip.com/ParamChartSearch/chart.aspx?branchID=1005

Then, in the "CPU type" box pick 8-bit AVR. Scroll across to the USB section and pick number of USB modules 1, 2 and 3, and then order by pricing. The displayed prices are only if you order 5000, so they will be a bit more expensive one at a time.

The winner on the list is the  ATmega8U2. It's about half the price, and slightly smaller than the ATmega32U4. It also has about 1/4 of the flash and RAM and half the eeprom and hardware timers, and no ADCs or I2C. But still enough for what you want I expect. Second place is the ATmega16U2, which is only 16 cents more, but has quite a lot more flash. If you don't want to stick with AVRs, then the PICs are even cheaper, but probably won't work with the QMK library.

Most manufacturers who have large complicated ranges have a similar tool, and they are great for finding the right chip.