In the case of new build each 30 premises passed share 2.488 Gbps downstream, 1.244 Gbps upstream.
This is physical layer rate not what may actually be carried as useful goodput.
After overheads, etc, you're closer to 2.1-2.2 Gbps down, 1.05-1.1 Gbps up.
Overbuild/brownfield FTTP same planning guidelines I believe though they may initially combine the premises so more are sharing the bandwidth and split the PONs as take up rises.
Either way no more than 30 connected premises per OLT port with the numbers above, so the many dozens of users are actually no more than 2.5 dozen sharing each port.
Should a PON saturate Openreach options are the same as VM's - add more capacity to the PON via XGSPON overlay and WDM or reduce the premises passed and in turn customers connected to the PON by placing half of it on a new port.
I mention XGSPON specifically as the burst optics required to support it are at the cost point where skimping and going with asymmetrical XGPON is not really worthwhile anymore, the only caveat is that using XGSPON over XGPON reduces the ports per line card in half right now though I'm sure that'll change.
EDIT: Just to clarify: yes, selling a gigabit upstream over GPON as Vodafone/CityFibre are is really pushing your luck however GPON shares bandwidth and congests more gracefully than the DoCSIS used in cable networks so some saturation isn't a problem.