Too Long; Didn't Read
There’s no such thing as optimal, just what turns more or less people into customers. Product marketing is essentially a service function, with the exception of when it runs launches and specific measurable programs like Analyst Relations. Some startups get to 100+ people and large sales team without acknowledging the need for PMM. The person you hire to come up with or execute a marketing strategy for GTM A will not be the same as GTM B. The work is done mostly by product and filled in by others.