You need at least two witnesses—one for the bride and one for the groom—but you’re free to have more. You might not want to exceed ten, but you can certainly include multiple friends or all your siblings.
If you’re torn between candidates, consider drawing names from a hat at a dinner party. That way, no one can claim favoritism, since the selection was left to chance.
Though we’ve talked about siblings so far, any adult relative—uncles, aunts, cousins, even adult children—can stand as witnesses. They should receive a special gift of their own, distinct from the favors given to other guests.