qmk_firmware/keyboards/handwired/numpad20/numpad20.h

#ifndef NUMPAD20_H
#define NUMPAD20_H

#include "quantum.h"

#define LAYOUT( \
    K00, K01, K02, K03, \
    K10, K11, K12, K13, \
    K20, K21, K22, K23, \
    K30, K31, K32, K33, \
    K40, K41, K42, K43 \
    ) { \
  { K00, K01, K02, K03 },  \
  { K10, K11, K12, K13 },  \
  { K20, K21, K22, K23 },  \
  { K30, K31, K32, K33 },  \
  { K40, K41, K42, K43 }   \
}

#endif