multitudo

Multitudo, multitudinis is a third declension feminine, built from multus with the abstract suffix -tudo. That suffix is a small factory: magnitudo from magnus, altitudo from altus, latitudo from latus, fortitudo from fortis - and every one of them is feminine, with -in- appearing in the stem from the genitive onwards. Multitudinis, never multitudonis.

It means both an abstract "great quantity" and a concrete crowd of people, and Roman writers can put the same sneer on it that English puts on "the multitude". It takes a genitive of what there is a lot of: multitudo militum, multitudo navium. In later Latin the verb sometimes agrees with the sense rather than the form, so you will meet multitudo ... clamabant, plural. That is not an error to correct, it is how the language moved.