I use 2. One for the singer and one for the audience. There is a large TV above the singer facing the audience. It was actually put in the bar for the purpose of karaoke. It is large enough you can see it from the back of the bar which is nice.
I can't imagine not having a monitor for the audience. I have so many people that sing along. I think it helps engage the audience and encourages people to come together as a group.
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~*~ MONKEY BUSINESS KARAOKE~*~ [/shadow][/updown][/glow]