The answer is all about the experience. An arcade is any area that truly reminds you of the Arcades you grew up with. For me it was a Gold Mine. Dimly lit, narrow, bout 4 rows facing each other(although 2 is enough), and great with noise. If your arcade had a raised walkway, you need a raised walkway, if you played pins heavy, you gotta have a pin. If music was prevalent, you either need a sound system or a jukebox. But you'll know when you have an arcade. I think I had an arcade at about 8 machines. That's when I got the feeling, I was in an arcade. But for those that don't know, here's the test.
1. Have you ever gone into your gameroom at night and turn all the games on and just look at them.
2. Have you ever coined up those games just to hear the background noises.
3. Does doing 1 and 2 evoke strong emotional responses.
If the answers to the above questions are yes, then you have an arcade.
But hey, that's just my 2bits.