I think that the Vet is more likely to use an ultrasound scan just to confirm a pregnancy rather than predict the precise number of expected puppies. The reason for that I guess is that sometimes developing puppies can be reabsorbed, or may not be so easy to detect if they’re ‘hidden’ up under the ribs.
I too have noticed some of the adverts for mobile ultrasound pregnancy scans - though you were fortunate that the lady lives local to you otherwise the incurred mileage costs can be quite hefty.
I suppose the other possible ‘plus’ is that a vet should have carried out a general health check on your girl at the same time, but on the other hand the mobile scanning lady may well have far more experience of doing these scans than the Vet. I’ve actually got no idea how reliable the mobile scans are - you’ll have to let us know!
We’ll all be waiting to hear how many puppies and to see some photos soon!