How to competently manage the size of the human resource in IT?

Question

Work organization

Probably, most companies in IT (and not only) often face the problem of disproportionate distribution of incoming tasks. Suppose today the sales manager brought 10 projects, and six months later, none. Thus, today we need 40 developers, and in six months there will be much less and no one wants to feed non-working people.
How to choose the optimal number of full-time specialists and how much to give to the side? And to outsource to teams or individual freelancers, in modules or entire projects, and how to work with them?
It is clear that the question is large-scale, so I clarify: "Tell me what books or methods are there for a complete immersion in the problem."

Answer 1

The question has nothing to do with IT. The answer is planning, understanding economics and project management. As in that joke - the cat got the mice. Mice came to the old rat and asked how to get rid of the cat - "And you all turn into cats!" The mice were delighted, and then they think, but how?..." So you just need to become a competent manager. Which, I'll tell you a secret, can be counted quite quickly in the country.

Answer 2

Your topic is not related to IT but to the organization of work.
If the manager scored 100 orders for the installation of plastic bags in a day, and then not a single order for a whole year, then the situation is the same as yours.
> offtopic, we are talking about IT and outsourcing is so popular in IT
Sorry, the computer is plugged into a power outlet. Does this mean that repairing an outlet is also an IT direction?

Answer 3

PMBOK
Theory of Constraints
MS Project

Answer 4

Constantly (conditionally) there should be as many people in the state as you can load (and accordingly feed) on a regular basis. All other fluctuations in demand are regulated by freelancers and outsourcing teams.
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