other none rare casualpolitewritten
大勢 — a large number of people
大勢 ・ おおぜい
A full written guide for 大勢 is on the way.
This point is mapped in the grammar tree, but its examples, formation table and usage notes haven't been written yet. The fastest way to learn it now is to meet it in real sentences inside Jengo.
See 大勢 in real sentences
Jengo shows 大勢 the way you actually meet it: inside real Japanese sentences, so it sticks instead of staying an abstract rule.
Study it in JengoSources Compiled with reference to A Dictionary of Basic Japanese Grammar. Editorial confidence: low.