say if you have 1000 subscribers, you offer 100 ports, you are assuming that out of 1000 max 100 can call concurrent. So this is only assumption.
if u have a voip gateway, u can only estimate this way as all busy call that cannot handle by the pstn will not be logged by the gateway. Hence there is no info on the gateway that indicates the blocking factor by the gateway.
At the PSTN switch side, there will be able to tell how offen the line to the gateway is busy.
so, normally take 1 out of 100 will make calls. But during promotion, the calls will surge and it will not be enough.