Crowd
A Large Number
noun, の adjective
A large force is a group of people. Just think about when this word was probably made up... a long time ago, and the biggest force you could have was a huge group of people. This group of people force is also known as a crowd, or a large number of people.
おおぜい
The reading for 大 is the おお reading (like 大きい). Nothing you can do about that but remember it. Then, 勢 is rendaku'd to become ぜい. Also something you'll just have to remember. Say this word a few times in your head, then you'll be able to come up with an idea of what sounds right and what sounds wrong so that you can figure it out next time this word comes up.
大勢の人の前で話す事に慣れていないので、今からもう手が震えています。
I’m not used to speaking in front of crowds, so my hands are already shaking.
大勢の生徒の出席を取る時、たまに一人か二人、名前を飛ばしてしまうことがあります。
When I take the attendance of a large number of students, I sometimes skip over one or two names.