input_map[p1_up]
input_map[p1_down]
input_map[p1_right]
input_map[p1_left]
input_map[p1_button1]
input_map[p1_button2]
input_map[p1_button3]
input_map[p1_button4]
input_map[p1_button5]
input_map[p1_button6]
input_map[p1_button7]
input_map[p1_button8]
input_map[start1]
input_map[coin1]

Most games don't use buttons 7 and 8, plus you can map start1 and coin1 to the same button or map coin1 to 'auto'.

For my 2 player setup, I have 2 4-way joysticks, 4 buttons for each player, plus 2 more universal buttons for coin1/start1+coin2/start2.

You can get by with just 4 buttons because you can map key-combinations to separate buttons. so I mapped buttons 1-4, then button 5 is triggered when pushing buttons 1+2 together and button 6 is triggered when pushing buttons 3+4 together. So you can get by with this setup, but if you plan on playing a lot of fighting games, I would recommend having 6 buttons per player, plus joystick, plus at least 1 universal button per player.