Plant spacing is mostly a matter of personal preference. Succulents can be planted right next to each other to get a bountiful, filled-in look right away. You may have to prune or re-pot relatively soon when the planting becomes overgrown or root-bound. To give the plants a little more room to grow and increase the longevity of an arrangement, leave about 1.0" of space between plants.
Many of the hardy or outdoor succulents are good at spreading and forming a tidy mat of ground cover. As a general guideline, plant Sempervivum about 4" apart to give them room to produce new chicks on stolons. Hardy Sedum spread even faster, about 1" per month in the growing season, and can be spaced less densely. Consider spacing a little more if you live in a mild climate (e.g. coastal California).