I think we should merge all of these into player-number and write a wiki for it. This tag should almost always be accompanied by a game-specific tag. (The exception there would be questions of the form "what games work well for seven players?".)
Nobody would ever search for extra-players without also searching for a specific game (what does it mean in isolation?), so searching for game + player-number works just as well. All non-solitaire games are multi-player, so that tag isn't particularly meaningful. As for two-player, why distinguish that if we don't distinguish one-player and other specific numbers of players? What's special about two?