Given that there are no devices (Wi-Fi or other) giving interference on the channel, I'd venture to guess about 30 clients. On just 11a/g, you can get away with 20. It all depends on the speed you want give your customers. Keep in mind that if your Internet connection is only 5Mbps, that will bottleneck you before Wi-Fi will.
What situation are you in? Coffee shop, WISP, guest access...