Well, are you walking around? Are people standing or sitting? How many people will be there? Do they expect you to entertain the entire group at one time?
Some generic advice:
If you are walking around, it's a good idea to have about 6 (rough number) effects. When you approach a group, you need a flashy opener, a strong effect or two, and a killer closer effect. The number of tricks you do will depend on how many people you have there.
If this is a new thing for you, go through your inventory of effects, and like Ace to Z-Magic said, "Stick with impromptu or extremely quick setup".
For example a couple of routines I use often are:
1. Appearing/vanishing silk (using a tt) -simple, opens well, and begins the routine script for me.
2. then my script allows me to brig out a deck of cards and I do "Jokers Wild"
3. then I will either do another card trick, or just finish using sponge balls. The routine I use with the sponge balls closes and finishes well, and ends my script.
another routine that I will often use if I have more time at each table is:
1 Joker's Wild
2 Biddle
2 quarter bend (Morgan's 'Closer')
3 Ambitious card with Sankey's paperclipped to finish. (I love this ending. After my opener, I hand someone a mini attache case to hold. After the last routine I get them to open it, inside is a folded paperclipped card, of course it turns out to be the card that they signed)
Remember your dialogue will bring your effects together. Don't just tell people that you are going to do another trick, but find a way to engage them, get their attention and lead them down a path where they will be blown away with your final effect.
And also remember to have another couple of your strongest effects for any really important tables/people. It singles them out and makes them feel important.
Well, hope some of that helps. Good luck!
justin