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P = f(A, N)

but now you are saying you want N such that P = f(A, N)

you know what A and P are

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from Wikipedia - A is traffic and N is number of servers -

Pw = ErlangC(A,N)

Pw is the probability that a customer has to wait for service

so are you perhaps saying that you want Pw to be restricted to some sort of value, .1 for example? That would mean there is a 10% chance of waiting for a resource, then you want to know what N is to achieve that probability