
For us, and we have had 2 small litters, one a singleton from the outset, the worst bit, pre-whelping, is sometimes they just 'sit', growing bigger by the day and so increasing the possibility of the need for a C.Section - such a pain for just one or two puppies! I'd tend to suggest you let her go to 63 or 64 days, and if nothing happens, get her into your vet who should examine her, maybe x-ray to see if there's anything there, and if so, give her a shot to start things. Be aware that occasionally a car trip does the same thing - so take a box with a warm pad in it etc. And somebody with you to watch your bitch.
Post whelping having just one can be sad because once mum has done her bit, they look so lonely in the box on their own. I carried one we had around with me and as a result, she was spoilt rotten.
I have never seen the point of scanning which only tells you there is a pregnancy, and only at the time of the scan. X-ray, which is quite safe when done towards the end of the pregnancy, is far more accurate.
You may have to adjust the amount of food you are feeding if she just has a few puppies ...... I remember the other small litter, our first which was really a disaster - 2 puppies, one DOA. As a result (and she did start to look for the dead one for a few days - heartbreaking) she really didn't eat anywhere near as much as I'd expected she would need to feed 'a litter' - according to the books. Clearly nature knew best and her needs to supply him were very low. He had a ball with the whole milk bar to himself - I did have to make sure he took from all the teats so she didn't run into mastitis - another thing to be thought about if you do only have a few puppies.
Having a small litter isn't ideal, but I don't think you need to worry unduly about that, other than the potential for a delayed whelping.
Add - Generally speaking I'd suggest your vet isn't out of line by telling you to wait for her due date - yes, they do all differ, but provided she's otherwise fine, no nasty offensive discharge etc., why not wait until she should have whelped before pressing the panic button?