The Collaborative International Dictionary of English v.0.48:

Multitudinous \Mul`ti*tu"di*nous\, a.
   1. Consisting of a multitude; manifold in number or
      condition; as, multitudinous waves. "The multitudinous
      seas." --Shak.
      [1913 Webster]

            A renewed jingling of multitudinous chains. --G.
                                                  Kennan.
      [1913 Webster]

   2. Of or pertaining to a multitude. "The multitudinous
      tongue." --Shak.
      [1913 Webster] -- Mul`ti*tu"di*nous*ly, adv. --
      Mul`ti*tu"di*nous*ness, n.
      [1913 Webster] Multivagant

WordNet (r) 3.0 (2006):

multitudinousness
    n 1: a very large number (especially of people)