Inspect capacity and conversion separately
Is the team missing opportunities because it cannot cover qualified demand, or are existing opportunities failing to progress? Check actual workload, territory coverage, buying activity and time spent on non-selling work. A headcount request should explain which constraint a new person would remove.
Account for the management load
A new seller needs onboarding, account context, clear expectations and feedback. Identify who supplies that support and how much time it requires. If current sellers are not getting useful coaching, adding another seller may spread that support thinner. Fix the missing management responsibility alongside any justified hire.
Write the conditions for hiring
Specify the demand, territory or coverage need, the available support and what useful ramp progress would look like. Review those conditions with the founder and the person who will manage the hire. Hiring may still be the right answer. The point is to connect the decision to an observed business need and a workable operating plan.