A full backlog does not tell you which extra person the team needs. A shortage of people may be real, but waiting for decisions, testing or approval can look similar. Before opening a role, separate the volume of work from the obstacles preventing completion.
Follow a few tasks through the process
Choose several recent changes and review when someone worked on them and when they waited. This is not about timing individuals by the minute. Look for a recurring point where work stalls. If completed changes queue for a single reviewer, another developer alone will not solve that problem.
Distinguish capacity from a specific skill
A team may have enough people for routine delivery but still lack a particular kind of experience. Bringing in a specialist for a narrowly defined problem differs from long-term reinforcement. Describe whether you need more throughput, new knowledge or cover for a critical role. Each goal produces a different brief.
Allow for joining the project
A new colleague needs experienced team members’ time, access and prepared tasks. Adding a person does not immediately add their full capacity. Account for onboarding, support, leave and operational duties. Where the outcome is uncertain, work with a range and state your assumptions.
Check whether the change helped
Agree in advance what to observe after the specialist joins: shorter review queues, for example, or a second independent owner for a particular area. A count of completed tasks can mislead without context about their size. Choose additional expertise around the problem it should solve, then keep checking whether that problem is getting smaller.