Each SC cabinets used 160 kVA = 640 kVA total for 4 SC cabinets. Depending on the utility company they may provide a 750 kVA or 1000 kVA transformer. At most Tesla sites they also weigh the need for future expansion and will on a site by site basis opt for capacity now verses adding it at the time of expansion to minimize impact to customers.
Are you talking about V2 sites or V3?
V2 sites typically use a 500 kVA transformer for eight stalls (4 144 kW charging cabinets) and a 750 kVA transformer for ten to fourteen stalls. The eighteen stall (nine cabinet) site in Springfield, VA has a 1,000 kVA transformer.
V3 sites typically have double the transformer size. An eight stall site gets a 1,000 kVA transformer. Las Vegas 24 stalls get a 2,500 kVA transformer. Many Trans-Canada Highway V3 sites are only getting six stalls installed, with room for two more (each cabinet can support up to four stalls and the sites are getting two charging cabinets).
There typically isn't additional headroom on the transformer side... but the opposite, where the transformer is smaller than the site's theoretical full load. Remember a transformer can supply more than its rated power for small periods of time, which Tesla has deemed acceptable for their use case. A site that's full all day will be limited by the transformer, as we saw in Asheville, NC a few weeks ago when the site was full for hours on end due to the Blue Ridge Parkway Tesla Rally.
This is a bit off-topic and has been discussed in other threads. The bottom line is that North Bend is a pretty standard V3 site with eight stalls, two charging cabinets and a 1,000 kVA transformer.