Recent readers of the Box may be aware of my crusade against the 8-man bullpen. I have nothing against it in principle, except that until September, the active roster for major league teams is limited to 25 players. When rosters expand, if you want a 14-man bullpen, I will raise a figurative glass of ale and say, why the heck not!
I'll add another caveat. On teams with weak starting pitchers wherein several starting pitchers have difficulty reaching the 6th and 7th innings, a three-man bench is defensible as long as at least two players and/or one regular are reasonably versatile.