Considering that the workstations are certified through their brand or maker for quality and reliability within their useful life years, and since you have a volume of 100, which is significant, if not, have it burned out to test its stability. Then, have the service sourced out on a cost-per-session basis. If not, employ 2 for 24 hours with 12-hour shifts instead of 3 at 8-hour shifts, which means 0 on top of the required manning of 300 or an additional 2 from your supplier or specialized service provider. If it is outsourced, make sure that the criteria and conditions are stipulated in detail, just like employing such skills. If not, hire 2 for monitoring, but the two can also be requested from your preferred vendor since your workstation volume is acceptable to make them your preferred vendor with a special arrangement. Since your industry is not based on IT troubleshooting, it would be best to have it outsourced for better, skilled, and reliable technical people pooled by the 3rd party service provider or supplier.
If the company has a good preventive maintenance schedule initiated by their outsourced service provider, the cost would be lower along with their downtime. Even if they have a backup, the time incurred between breakdown and replacement is already considered a loss of opportunity, e.g., waiting time. If there is a preventive maintenance agreement with a 3rd party service provider, they will have advance knowledge of the workstation conditions rather than employing additional people waiting for a breakdown. Still, there are costs involved in waiting.
1. Do not employ just because a machine "MIGHT" have a breakdown. Implement a systematic preventive action, and the management will be informed ahead.
Do not hire and develop people who do not have growth opportunities in a company that does not relate to your product or service. Eventually, they will seek a company where they can grow or simply leave, and then you will hire another set of trainees and allocate costs for their training. It is not about disliking hiring people specific to a certain number but about hiring people who truly add value and focusing your management on services to your customers rather than on your infrastructure, which can be outsourced.
Hence, the 3rd party service provider must be prompt in addressing any system concerns, including the 2 assembly, disassembly, and installation personnel, with a clear and black & white MOA on service provision that addresses all management concerns. If possible, a risk assessment and preventive maintenance plan should be in place to prevent situations leading to a loss of opportunity. If you add two from your supplier as part of your in-hours requirement, they are not employed by your company but are service personnel from your preferred vendor. If someone is on leave, with or without notice, it will be your supplier who fills the gap.
If any of the 100 users per shift is absent and you employ more for the "JUST IN CASE" situation, it will be too costly in 1 year. Impose a policy regarding absenteeism, proper leave filing, and set up an on-call system rather than paying standby employees. Provide rewards, recognition, and appreciation when employees comply with expectations. Not all employees are self-driven.
If the company needs 100 per 3 shifts, then it's 300 since the rest are outsourced. They are still not employed.
To keep the 100 workstations busy, invest in company-quality, checked, and certified for quality, reliable, stable, high-speed PCs and dispose of them according to schedule whenever there are urgent needs or problems. The service provider will handle it for you.
Hence, focus on your company's service to your customers while controlling waste. :) Maybe... just maybe... :)