Calculation Of Man Hours In Service Support Model

Hi Seniors,
I am working on a project which is not live yet. I have some data like size of the team, no of tasks and average time spent on each tasks. This is a simple calculation but what I dont have is the work for which I dont know how to calculate man hours like sitting on the system and monitoring throughout the shift.
there are 3 shifts per day in a 24x7 environment.
Can anybody share the best practice to calculate the man hours here.
My main motto is to increase the size of team which I believe is less at this moment.

Dear Friend,
Three shifts a day clearly means an employee is subjected to work for eight hours a day in any of the shifts alloted which is 40 hours a week in a five day environment and 48 hours in case of six days work environment. My concern, what's the purpose of calculating man hours; is it for paying them salary or is it for productivity purpose.

Dear Rathi,
I second Unmesh. The calculations alone might help you create new positions, list the following down:
Downtime for the task , if the member is not present
Time taken for the talent to be trained, both classroom and on-the-job, before being billable
Bottle-necks and dependencies to each role
Existing budget vis-a-vis to billable ratio

Yes, New positions needs to be created since I see the volume of work but not sure how to quantify or showcase it to the higher management. If we have any such formula to calculate, please share.