It depends a lot, but some useful metrics are how many different actual systems there are to maintain? The more the higher the maintenance burden usually is.
Taking care of PC and end-user questions is a different story imo which deserves a helpdesk service specialized in user-interaction and expertize on the end-user applications. In smaller shops like this one, you usually promote certain end-users as system owners for their "favourite", responsible for answering questions and solving end-user problems - leaving you with less traditional helpdesk burden.
Depending on your environment and service level needs, a 100 people company could get by with anything from one part-timer to ten dedicated helpdesk, system administration and network engineering personnel.
Ask management what kind of down-time is acceptable. Is it ok for employees to have to stop what they're doing and get help whenever a desktop breaks down and is serviced? Or should you do proactive monitoring and fast near-non-interrupting replacements? Then take that question on to the back-end systems.
Perhaps a contractor job would be of value where you get a team of experts installing and configuring a more automatic and enterprisy environment for management and monitoring?
In my experience the burden increases exponentially after the first 30-50 people network and when it hits ~100 you could face the need of 4 times the IT support and maintenance needs if the environment doesn't scale well, isn't automated enough and so fourth. After that though I feel the curve usually relaxes a lot.