For managed IT services to be of value to your company, you should have at least 10 workstations/end users and at least 1 server.
The maximum size depends on your company’s business model, number of locations, strategic initiatives, growth plans, etc.
For companies under 200 end users, an MSP can be the total IT department (CIO, service desk, network administration and centralized service) or just act as a supplement to in-house IT staff with ala carte services and projects.
For companies with more than 200 end users, we usually see businesses hire an in-house technology engineer, but still keep certain parts of IT strategy, Business Continuity, Cybersecurity and Network Patching and Monitoring outsourced depending on your needs.