The main idea is that as long as calls, all calls are going to a same number and you have a strong infrastructure that sends or distribute those calls to agents that can be seated at a central site or can be distributed on different locations on the city or world wide, you need the minimum amount of agents. If on the other side, you have different locations not integrated to which callers call on different numbers, then you need to dimension number of agents per site, and in that case the total number of agents increases depending on the level of service you want to give. It is different to dimension 5 call centers that gathers lets say 300 inbound calls per hour, than to get one call center with 1500 calls per hour.
I would suggest you to check on Erlang C tables as a start in order for you to realize differences in staffing…