desktop support is different from server support and depends on os so for a vendor neutral guess
groups
desktop servers/network databases electronic devices/pbx
desk lead and server/network/lead database lead controls lead
reporting to tech services head
now security could be handled by server/network people(since that is what I do I prefer it that way.
staffing ratios are as addy pointed out entirely based on configuration and control.
Also be aware that your ~15% experts will be >20% of your problems.IMHO
thanx,
bill