I think we have to identify three stages in the "financial" life of a startup:
Number of employees: 2 co-founders, "pre-financing", little or no income
The startup generates few financial flows (whether incoming or outgoing) and it is sufficient to have one external accountant, who works with one of the co-founders. You have to estimate between €1,500 and €2,000 in fees per year.
Number of employees: 20–25 people, post seed-funding/series A, little income
The company also keeps its external accountant. However, an Administration and Finance Manager is often necessary to link the operational team to the accounting department, to help the CEO with his budget and reporting, and to take on legal and human resources tasks.
This profile therefore requires efficient multi-tasking and is essential for the proper financial and organizational operability of the company.
Number of employees: 50 people, post Series A or B, 1.5+ million euros/year turnover
A true finance function is needed at this stage. Public accounting can remain outsourced. However, an internal team is generally set up with 2 or 3 people: an Administration and Finance Director, a Management Controller and an Accountant. The team's responsibilities are: reporting (reporting all operational team information, revenue, expenses etc.), management control, budget planning (every 3 to 4 months), as well as transversal projects related to the company's growth: fundraising, monetization, opening new countries for example.