That's a good question, and one that we spend a lot of time answering in Head First PMP -- there's a whole chapter on human resource management.
The short answer is that, as Kengkaj Sathianpantarit pointed out, you need to know the scope. That is, you need to know the project scope, or all of the specific work that needs to be done in order to complete the project. You also need to know the sequence of those activities, because if one activity depends on another, then you can't hire people to do them simultaneously. You also need to understand your budget and cost constraints, because that will restrict how many people you can hire. And if you've got a resource calendar or any resource constraints, you need to know them as well.
We go over all of that in a lot more detail in the book.
Author of Learning Agile, Beautiful Teams, Head First C#, and Head First PMP (O'Reilly)