I'd say to stat up 7 PCs myself. The less the players have to select from the easier/faster it will be for them to choose, and the more able you'll be to remember the PCs powers in order to help out when they have questions.
I found in other presentation games that having lots of pregen characters enhance the experience a lot. One of the fun parts about RPG is character creation. That is impossible in a presentation game, but having options let the players feel the character more of their own. I have found specially important to have several characters of both genders on all roles.
Also it diminishes the chaces of two players wanting the same char, or having good second options. Also, either if only 6 chars are finally used, the players get the idea of the variety of different chars you might create and play.
And finally, it makes it more fun for me, since I can´t know for sure what combination of characters will the monsters face, making the adventure more intresting and fun to DM.
Of course, when selecting characters, I dont give them any character sheet, but a sheet with the description of the race, class, a general background on the specific char and a good cool image. No numbers on this stage.