qmk_firmware/keyboards/4by3/keymaps/numpad_horizontal/keymap.c

#include QMK_KEYBOARD_H

const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
	[0] = LAYOUT_horizontal(
		KC_P1   , KC_P2   , KC_P3   , KC_P4   ,
		KC_P5   , KC_P6   , KC_P7   , KC_P8   ,
		KC_P9   , KC_P0   , KC_BSPC , KC_ENT
    ),
};