For me, that number is 4. I own and use more than that, but three is my minimum.

1) the more important for me right now is a looper pedal. I got my first looper pedal in 2006 or 2007, and since then it has become a vital practice tool. I think everyone should own one.

2) Tuner pedal. I use a Boss TU-3 but there are plenty of good options. There are many ways to tune a guitar including some clip-on options, portable tuners, and cell phone apps, but a Tuner pedal is more than just a tuner. It's also a mute pedal; It's convenient for when you're switching guitars or unplugging/plugging in for whatever reason.

3) Delay/ Reverb. I can't decide which is more important. I just need one or the other but if I can have both I take both. Reverb pedals become less important if my amp has a good reverb already on it.

4) Overdrive pedal. I just need some kind of boost. I'm not too picky. I have a Tube Screamer and I'm thinking about adding a second Tube Screamer to my Effects Chain. It's pretty versatile.