Good question: much more than the 10k advisory from F360. I believe in the range of 3 million, more than that and vcarvepro takes too long.
For the uk i have used the 50cm square DSM or 1m square DTM from DEFRA which are 5km x 5km tiffs at 32 bit float resolution. And like reading proper map books (vs satnav) i find the area i want cuts across 4 pages, in my case 4 raster tiffs. So I merge them to one, reproject to a reference system common to OpenStreetMap (so i can use there data and convert kml files from google earth) and then clip to the area i need.
I have just run a map of Mallorca that was 20m square resolution but still needed 4 file referenced.
I find modelling with a lower resolution proxy of the model efficient, then go hi res for the final model with smoothing to avoid faceting in the 3d relief.