At twenty people the job is deciding what not to do, and that is a senior judgement being asked of whoever is hired. The common structure, a first marketing hire two years out of university reporting to a founder with no time to direct them, sets that person an impossible task and then concludes that marketing does not work.
What the role actually requires
Someone has to choose the segment, write the positioning, pick two channels and refuse the other six. That is strategic work, and it has to happen before any execution is worth doing.
A junior hire can execute a plan extremely well. They cannot be expected to produce the plan, and asking them to is the most common way a first marketing hire ends badly for everybody involved.
The founder cannot keep it
Founders usually hold it too long, because they hold the positioning in their heads and have never had to write it down. It works until it is competing with fundraising, product and hiring, at which point marketing becomes whatever is left on a Friday.
The signal that it has to move is not headcount, it is that the founder can no longer tell you what went out last month.
Structures that work at this size
Senior direction plus junior or outsourced execution is the pattern that holds. That can be a fractional head of marketing, an agency carrying the strategy, or a founder who genuinely protects a day a week for it. What does not work is execution with nobody owning the decisions.
Whichever shape you pick, one person has to be accountable for the number. Split across a founder, an agency and a coordinator with nobody named, it is nobody’s.
What to ask before hiring
Ask what the first ninety days would be, and listen for whether the answer starts with research and a decision or with a channel. Candidates who open with "I would start posting on LinkedIn" are describing execution of a plan nobody has written.
Ask what they would refuse to do. A marketer at this stage who will not name things they would stop has not understood the constraint, which is time rather than ideas.