A game for two. Should I make it playable by more?
It depends, probably not at first.  Most truly enduring games (Chess, Go, Reversi, even Magic: The gathering, etc) were originally for two players.  But also most of them have variants that arose later that allow 4 players.  

In short, if adapting for more than 2 players is non-obvious, then it is probably not worth the initial effort.  But if the game becomes popular and there is a demand for a multiplayer version, then it is likely worth doing at that point.  The changes could be fairly small, or they could be substantial (Magic required only small tweaks to produce multiple multiplayer variants, but most multiplayer versions of chess require a different board and rules tweaks.)