Well, the first advice is not to get your hopes up. From what I’ve seen and others I’ve talked to, selling photos is a tough dollar. Most people will look, admire, and then think “I could take that”. We’re also in the middle of a recession, and art is generally not a fast mover when times are tough. That said, it is a nice ego boost to sell some of your work, and juried art festivals do tend to attract the well-heeled.
Photos are usually sold either as bin art (matted in acetate envelopes) or framed. Pricing on bin art can be fairly low because material cost is much lower. Depending on the quality of your mats, the size of the pictures, and your desire to, you know, actually keep body and soul together, you can price them anywhere from $15.00 to $100.00. I think $25-$40.00 is the sweet spot. You want to keep it in impulse purchase territory.
Framed pictures are another story, mostly because frames are stupidly, ridiculously over-priced, even if you do it yourself. Materials for the simplest, black metal frame is probably around $30-$40, depending on whether you go with museum quality non-reflective glass or just ordinary hardware store glass. You do, by the way, want to stick with very simple frames for shows. Simple black or silver frames go with just about any decor, and someone who really cares is going to reframe the picture anyway (or buy the bin art and have it custom framed). Most framed photos that I’ve seen in art shows start in the $150.00 range and go up from there.
Obviously if you’re selling your work, you want to make sure that you’re producing archival quality photos. Most ink jet prints are very good nowadays (I haven’t seen any fading in my Epson prints), but (assuming you’re shooting digital), you’ll want to make sure you’re using decent inks that have been rated for longevity. That probably means using OEM inks, rather than refills.
As for how many – your best bet, if accepted, is to ask the organizers or previous participants to get any idea of how much business they did. You’ll probably be assigned a booth with a given amount of wall space to cover, so that can be your guideline for framed work.
Finally, don’t get discouraged if the jury rejects you. There’s a lot of competition and figuring out what a jury likes or doesn’t is very much a crap shoot.
Caveats – I’m only on the periphery of the artistic photography scene – I’ve had a few photos accepted for shows but have never sold one.