Yup, I was concerned - and did take her for an ultrasound.
My 2-cents: An ultrasound can be done earlier in the pregnancy to confirm pregnancy (if you need to confirm early). However, don't count on it for giving you an accurate count on the puppies.
For a more accurate account, we x-ray 6 or 7 days prior to due date so we can count skulls and spines. For our last litter (Sept), we could clearly see 10, but there were shadows so our vet said to expect up to 12. The vet was right. There were 12.
Why x-ray? In addition to finding out how many pups, our vet checked size of pups. She looked for uniformityin size and measured skulls to see if there could be problems.
If spending $$ for both tests is too expensive for you, I would recommend the x-ray -- especially if you think your bitch is carrying a lot of pups. Also, don't wait until the last minute to x-ray as the pups could be born prior to the due date by 3 or 4 days. "Due date" is an estimate based on averages; it is not in concrete. Our experience has been that if the bitch is carrying over 9, she is probably going to be early.
Re: natural breeding thoughts. When your bitch was bred can be a factor in pup size. Our practice is to get at least 3 breedings over two days -- hopefully morning, night, morning... or night, morning, night. Sometimes this is not possible and we have had to let 24 hrs. lapse between breedings which means a 3 or 4 day spread.
Our theory is... don't stretch out natural breedings too far apart. Why? Conception could be on the first breeding and the last breeding. If stretched over 3 or 4 days, the size of the pups could vary because the first conceived pups and the last conceived pups are going to be whelped all at the same time resulting in bigger pups and smaller pups. Our theory could be wrong, but that's my 2-cents as why you might plan your natural breedings closer together than waiting for 24 hrs. + in between.