There is no universal headcount that alone proves eligibility. The staffing analysis depends on the actual role and organization.
Budget for the real operating model
Assess who performs the work and how the manager is supported. Separate staffing expenditure from government charges and professional review fees.
The spending sits in retrieval and analysis: obtaining archived payroll and pension records, translating foreign documents, and paying for counsel's review of a history with several possible readings. Government fees for an L-1A petition are fixed by the USCIS fee schedule and depend partly on the employer's size and on whether premium processing is requested; check Form G-1055 rather than relying on a figure someone remembers. The expense easiest to overlook is the one the petition itself depends on: an executive role only exists if other people are doing the operational work, and hiring those people costs wages before the U.S. operation produces revenue. Direct the budget instead to places where evidence is genuinely missing: retrieving payroll history for the qualifying foreign year, obtaining certified corporate records establishing the relationship between the employers, having position descriptions written by the people who supervise the work.