It's not up to you
The Executive staff should determine how many persons in what positions are required to run the business. Their job is to prioritize which services are the most important services to run the company. Your job is to hire the best persons to meet their priorities. I suggest that you list all of the services, regardless of priority, you believe will be required to run the company 12 months from now. Go crazy. You need networking, phones, e-mail, web services, security, databases, user support, financial apps, etc. Document the role of each service to the business and the impact when the service is down (i.e. when the network is down you don't get e-mail, Internet access, customers can't access our web sites, etc). Let the Executive staff prioritize which services are most important to the business. Now design the IT staff to meet the requirements set forth by the Executive staff. Of course it will be bigger than they expected... But now it is on their shoulders to reduce the persons instead of on you to convince them to hire more.
