How exciting to be expecting puppies. You should be able to feel the puppies at this stage, but people are right that it can sometimes be hard to feel them in larger breed dogs. Sometimes even veterinarians can't palpate them well with years of experience. Other signs like mammary developement, enlarging belly and vulva can really let us know that a dog is pregnant. If you want to know how many there are (always a good idea to make sure when she gives birth that she has passed them all), you might consider a trip to your vet for xrays +/- ultrasound.
You may already have most of this information, but just in case you don't, here is a great link on what to expect when during your dog's pregnancy and whelping. It even gives great information on what problemscould occur, what to do, and when it might be necessary to contact your vet.
I hope this helps. If there are any further questions, just reply and I will get back to you as soon as possible.