According to the lists of spacewalks (1965-1999, 2000-2014), spacewalks have all consisted of only two people. The only exceptions are the very early space missions, where only one person went out, and one spacewalk on STS-49 which had 3 people. It makes sense that you would want more than one person to go out, in case something happens to one of them, and you would still want people inside in case something goes wrong.
Is there a written protocol for spacewalks which determines how many people must go out and how many must stay inside? Or do the various space agencies just play it by ear and try to minimize the number of people who go out?