How much incremental traffic would an additional SEO analyst bring if we were to hire one?

I was asked this question and TBH, I am not sure how to answer it so I am reaching out to this community for some guidance.

If you work for a large int'l company and SEO is a major source of traffic but the team is stretched and as a manager you see this and have requested those above you to give you more resources. The only problem is, how do you quantify the incremental traffic to justify the headcount?

I find it difficult because while SEO provides direction/strategy, the execution is done by other teams. e.g. content writers, web production, translators, etc.

One possible way is to estimate keyword TAM and the company's SOV and assume the FTE will lift X%. But how do you estimate what this X% is?

Anyone has experience with this or have ideas? Thanks