Multitude, army, host, legion mean, both in the singular and plural, a very large number of persons or things.
They do not (as do the words compared at CROWD ) necessarily imply assemblage, but all of them can be used with that implication.
Multitude stresses numerousness with respect to what is the standard for or the test of numerousness in the thing referred to; thus, in "that child always asks a multitude of questions" and "I never saw such a multitude of books before in one house" multitude obviously refers to a much smaller number in the first than in the second illustration.
When applied to a group of persons taken as a whole, multitude suggests an assemblage of a large number of persons, but multitude with a definite article suggests the masses of ordinary people or the populace.
Army usually adds to multitude the implications of orderly arrangement without a suggestion of crowding and often, especially in clearly figurative use, a progressive advance without any suggestion of halting or gathering.
Host has for its primary implication numerousness. It may mean nothing more, but it may suggest more strongly than any of the other words a concentration in great numbers of the thing referred to; in such cases it often connotes an impressive or striking array.
Legion in general use retains little suggestion of its basic application to the chief unit of the Roman army and but little more of its scriptural uses; typically it applies to an indefinitely or incalculably large number.