A few ideas. One, take as many as you want to sell. Two, if you have too few, you won't make expenses. If you have too many, you'll look like you don't sell many. 5 is too few. 20 is too many. Three, try and have a variety of price points represented. Don't bring all high end or low end stuff. Spread it out. Four, have one piece on the table that represents your very best work. Try and have at least one knife that's more impressive than the rest. Five, it's not a bad idea to have a bright colored knife or two. I typically put a few low end EDC sized knives on the table in green, orange, or pink G10.
Knife shows these days are about more than sales, as most of us could sell more knives sitting at a computer in our boxer shorts. Yeah, we hope to sell some, but we're really there to network and learn from other makers. Talk to the guys whose knives you like. Ask them for input on your work, on pricing, on shows in general, and then listen to what they tell you, and then do it. Also, many times a show will have supply dealers, so come prepared. Stag, ivory, pearl, wood blocks are often best picked in person, and you don't have to pay shipping if you just want a piece of pin stock or a few corbys.
Good luck!