David, I think you are on the track to an answer yourself. You seem to have calculated your man-hour requirements - from that you can arrive at a figure for staffing.
But, I know that things in big companies aren’t always so simple! I suspect your situation reflects what many web managers have to cope with. A large fragmented organisation with multiple departments controlling different aspects of site support. If I am right, you probably have no central team charged with overall site governance (or if there is, it is somewhat toothless).
Loose management structures make manpower planning more difficult, because cumulative activity is harder to measure. For example, if 10 different teams are doing similar work at various levels of intensity, overall effort is likely to be under- or over-stated.
My ‘idealistic’ reaction is to review & consolidate management structures first and then set about planning manpower. Only when definite management systems are in place can staffing be planned properly.
My ‘pragmatic’ reaction is to establish if all the activities of site management/maintenance are being effectively carried out, as things stand. If so, staffing is adequate. If not, more are needed. Just how many more depends on how much time (manpower) it would take to get things right.