I dont have any document like that you required, but will try to give some information about this matter.
If you have the given number of voice channels (N), you should choice your preferred blocking probability (GOS, range from 1%-4%). From these values of N and GOS, based on Erlang table or Erlang calculator you will get the traffic capability in Erlang (E).
Then you have know the average time (in minutes) that each subscriber occupies the voice channel per hour (T), by mean of forecasting or statistical data. Hence you can determine the average traffic for each subscriber (Es) by the following formula: Es=T/60. And at last, you will get the number of subscriber = E/Es.
Below is the example for easy understanding:
Let say N = 30 channels, GOS = 2% -> E = 22 Erlang.
Let say T = 1.5 minutes -> Es = 1.5/60 = 0.025 Erlang.
-> Number of subscriber = 22/0.025 = 880 subscribers.