"What's CFs core network capacity?"
I've not seen anything in the public domain about Community Fibre's infrastructure (other than some documents about the IPv6 implementation).
One might imagine that the "optical" part of the network ends up on some active equipment in an exchange (or similar building). At this point the data from each subscriber is aggregated onto links that (directly or indirectly) end up at at some infrastructure in the large Internet data centres.
A look at "PeeringDB" suggests that there are at least 3 significant sites, 2 in Docklands and one in Slough and one might expect leased circuits from the location of the local active equipment to at least two out of the 3 (to give some degree of resilience).
While individual consumers are offered what seem to be very fast connections, obviously few produce or consume lots of data all the time. Hence the connections "out" of the internal network and at points within the network can have some degree of diversity applied.
While exactly how much diversity can be assumed is clearly part of the design process for an ISP, a quick look at my router suggests that on my 150Mb/s connection I average 1.3Mb/s over a few days (which actually seems surprisingly large).
Going with that figure (and discounting the possibility that my calculation is flawed!) then 100:1 would be OK for lots of people like me, although in practice there needs to be an allowance for peaks in the data flows.
Back in the days of ADSL I recall that a 50:1 contention ratio was mentioned. This may well still be the design target for consumer services today.
To state the obvious, the availability of fast connections allows consumers to use more data so whatever is designed needs to allow for both more customers and more data per customer.
I'm not sure that I've made much more of a contribution then
@Pheasant's "Enough" but my practical experience is that I never notice any sign of general network capacity issues with Community Fibre.