
I have only ever used scans to confirm pregnancy as most vets can be wildly out, other than detecting a small litter of one in breeds where this would be a problem.
When all is said and done the number is not relevant as they have yet to be born and reared, and of course pups can be still born or die soon after, so I don't like counting my chickens.
It can be useful after whelping to confirm that all pups have been born if in doubt.
If the Vet is certain after palpating then I personally woudln't bother with a scan.
My vet palpated at around 21 days, I think, then we scanned at 32 days also. Th evet was sure after the palpation, but I still wanted her to scan so I could see the pups and hopefully get a rough estimate of how many pups, as this was my first whelping, with my mentor on the phone but 1 1/2 hours way. The vet was very honest about not being able to give an acurate estimate of pups, but I asked anyway. She said probably 7 or 8, but could equally be 6 or 9, so quite a broad figure, but she had 9. I did also weigh my bitch regulalrly throughout her pregnancy though, and just before whelping she had gained 9KGS, which was all pups, not body condition, so I did guestimate that that would be 9 pups at lb each plus about lb per pup for fluid/placentas etc. Of course, I might have just been lucky!
It was nice to have the scan though, to see the pups little heart beats. My bitch loves any sort of attention, so was fine with the scanning, so I didn't see the harm.