Hi Lila, we have 3 system administrators who are responsible for maintaining/governing the system configuration and design of various solutions + support. We have 2 super admins (group admins with all privileges) who are training to become system administrators (3-6 month process) - a team of 5 globally (APAC (2) - EMEA (2) - AMER (1)).
We have an extended team of 150+ Power Admins (group admins with fewer admin privileges) who manage support queues at a market level (33+ markets) - their main role is user/project support/maintenance. The Power Admins can interface with the system administrators using a support queue, and the system administrators get roughly 100 - 200 tickets per month.
We have roughly 7,500 users; with plans to grow to 10-15k this year.
If you're working in the US only; I would recommend starting with 2 expert system administrators for business continuity and 2/3 Power Admins (Group Admins) per Home Group. If global, you may want to consider the regional presence of system administrators.
Hope this helps.