The number of machines an individual staff member can support will decline as the differences among the supported machines increase. Expectations for service. Skill level of IT support staff. The more highly trained the IT support staff, the fewer required to provide a given level of support. Critical mass, division of labor, and economies of scale.
Every support organization has to cover a range of skills.
This requires a critical mass of support professionals with a wide enough range of skills to cover a plethora of IT issues, routine and complex. A support organization that falls short of this critical mass may incur high overhead to address IT issues and quality of service may suffer.