There is no minimum distance stated anywhere for ranged attacks, though logic would dictate it isn't zero. Since creatures can't normally occupy the same square, if using a grid, it stands to reason that all creatures will be at a distance greater than zero from one another as long as at least one creature wants to be.
That is, a creature could always move, say, a foot within his own square and be at >0 feet from another creature and make a ranged attack.
Though I wouldn't object too much if a DM said you couldn't use a ranged attack on a creature within 5 feet of you.
That is, a creature could always move, say, a foot within his own square and be at >0 feet from another creature and make a ranged attack.
Though I wouldn't object too much if a DM said you couldn't use a ranged attack on a creature within 5 feet of you.