Familiars? Animal companions? Intelligent swords?
These are options for characters strictly in accord with whether another character is needed, or clutter.
True, a novelist can experiment with throwing one in to see if the new character shakes things up, but then he has to remove it again if the effect doesn't work. A party consisting of a rogue, a fighter, a cleric, and a wizard is already getting interesting to juggle. Throw in the fighter's magic sword, give the wizard a familiar, give the cleric an animal companion because of his domain, and things can easily get out of hand. . . .
Though, of course, it may be the human(ish) characters who have to go.
But who stays and who goes is driven by plot and theme and such like. A familiar makes an excellent foil to the young wizard, so much as to be almost cliche. A ranger who is driven to meet the others after a long, desperate, and solitary trek would lose his edge if he had so much as the companionship of a wolf on the way. (Killing the wolf at the beginning might work, or in the middle if the trek needs to rise in danger, but not at the end, where his solitude is about to be assuaged by companions.) On the other hand, another ranger might be better accompanied by the wolf, and his intelligent, empathetic sword that can not speak but can, after the ranger has avoided something it deems a duty, rest in its sheath radiating a disapproval colder than the winter they trek through.
But they should get added and removed according to the need of the cast, and the rest be worked around that.