How many effects should you have ready to go before you head out and perform for new people? How many tricks to a routine?
The limiting factor is usually your environment and your timeframe.
If you're out on the street, you can get away with having three tricks for a group. You use one to open, then do something more involved, then cap it with a final blow-away effect. But what you need to remember on the street is that you don't have just one group at a time. People will follow you. People will go get friends and come back. So you still need enough material that those people will still see new tricks.
In that situation, I'd like to have four openers, six intermediate tricks, and at least three closers. The opener doesn't even have to be a trick. Here are the ones I currently use for impromptu stuff in public:
1. Fan the cards, facing toward me, and ask the spectator to pick any card.
2. Underspread force the bottom card.
3. Blind shuffle the deck and hand the (known) top card to the spectator.
4. Hand a known card to the spectator.
Immediately after this, I name the card, and then ask "do you know how I knew that?" - explaining how I know in every case. (I don't explain the underspread force specifically, just the concept of forcing a card.) This makes me look like a rebel and one of the "bad boys" of magic.
Sometimes the spectator will already know the answer and explain it himself. In that case, I congratulate him, and explain that it's always more challenging and fun to perform for someone who understands how card magic works.
Either way, my purpose is to break down the us-and-them barrier. There's an inherent conflict in any performance where you're the magician and they're the audience, and you're competing. By presenting the performance as more of a conversation, you can defuse a lot of jerk behavior.
The intermediate tricks I use are basic stuff; a triumph, a reversed card location, a holdout, simple things like that. Nothing particularly fancy.
And then at the end, I have... well, still basic crap, really. I'm not terribly good at card magic, and I don't buy tricks, so I'm currently without even one good closer. I'm working on developing one, but it's not easy. Since you have a few purchased effects, you'll undoubtedly have an easier time of it.
I also don't do street magic
per se. I just do occasional impromptu performances when I'm out somewhere waiting for someone or something. Most of them happen in bars, restaurants, or the lobby of an office building, so I tend to have a table available.