How many agents do I need on the phones?
Enter the calls you expect, your average handle time and your service level target. The Erlang C model works out the agents you need, then adds shrinkage for the rota.
How it is worked out
Workload in Erlangs is the calls in an interval times the handle time, divided by the interval length in seconds. 120 calls at 300 seconds in 30 minutes is 20 Erlangs: you need more than 20 people just to keep up.
Erlang C then works out the chance a caller waits, for each possible headcount, and from that the share answered within your target time. This page adds one agent at a time until you hit your service level.
Agents on the phones is not your rota. Shrinkage covers breaks, coaching, training and absence. Divide by one minus shrinkage to get the people you need to schedule. Watch occupancy too: above about 85 to 90%, agents burn out.