The Collaborative International Dictionary of English v.0.48:

Multiplicity \Mul`ti*plic"ity\, n. [Cf. F. multiplicit['e].]
   The quality of being multiple, manifold, or various; a state
   of being many; a multitude; as, a multiplicity of thoughts or
   objects. "A multiplicity of goods." --South.
   [1913 Webster]

WordNet (r) 3.0 (2006):

multiplicity
    n 1: the property of being multiple
    2: a large number [syn: numerousness, numerosity,
       multiplicity]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "multiplicity":
   Proteus, abundance, allotropism, allotropy, barrel, countlessness,
   diversification, diversity, great deal, her infinite variety,
   heterogeneity, heteromorphism, infinitude, innumerability,
   lashings, lot, manifoldness, manyness, mass, mess, much,
   multifariousness, multifoldness, multiformity, multitudinousness,
   nonuniformity, numerousness, omnifariousness, omniformity, peck,
   plenty, polymorphism, power, profuseness, profusion, rifeness,
   shapeshifter, sight, swarmingness, teemingness, variation,
   variegation, variety, variousness