How efficient your staffing has to be depends significantly on how well you addressed this area in the contract with the customer.
Many organizations that hire full time staff make decisions once per year on whether their headcount is too many, just right, or too few. It is like the Goldilocks story but without the happy ending if too many, leading to layoffs, reductions, destaffing, or whatever your organization chooses to call it.
Organizations that hire temporary staff sometimes have a core of full time staff which they supplement with temporary staff for surges in customer demand.
If serving an external customer with a contractual agreement between the two parties, you need to pay attention to the terms and conditions that address when the customer authorizes spending to begin. If you staff before this time, you may have to eat the costs. If you staff after this time, the customer gets frustrated with your responsiveness to their courseware needs.
Leave a buffer to handle variability where possible.
Understand all the relevant parameters for staffing.