Indeed agree with all of the above , when you say support do you mean if the system dies the engineer has the skills to rebuild the entire estate or is it day to day admin for moves and changes .... I would say a minimum of 3 for admin due to the shift pattern and holidays etc , it really depends on what skills the engineers require etc , from there maybe better advice can be provided.
APSS (SME)
ACSS (SME)
ACIS (UC)