21 workmen in each shift will be ideal, and 7 workmen will be working as relievers. However, if you put 20 workmen in each shift and 10 as relievers, then you can control absenteeism and avoid overtime because these workmen will work 3 days in shifts and 3 days in general shifts. Whenever any workman is on leave or absent, you can utilize these relievers' services in shifts instead of general shifts.
Prepare duty roster for workmen as 3/2/1, i.e., a workman who is deputed in the 3rd shift will perform his duty in the 2nd shift the next week after the weekly off. The 4th workman (Reliever) will perform his duty in the general shift for the first 3 days and then keep him in the shift from the 4th day onward during the weekly offs of other guards and utilize him as a reliever during the absence or leave period of other workmen. This method will help you partially resolve your problem, control absenteeism, and minimize overtime.
Find attached duty roster for your ready reference and easy understanding.
Regards,
Suresh