Oh! How exciting!!! I would say plan on 4 or 5... then you'll be over prepared instead of under. I think I see 4 spines with possibly a 5th. But again, I'm no expert.
Tenley had her puppies 2 weeks ago. I took her in on day 50 because that is what my vet wanted, and we saw only 3 spines with maybe a 4th. He told me to bring her in again in a week, just so we could make sure. And BOY am I GLAD we did! She had SEVEN puppies in there!!! It's amazing what that week did for the x-ray.
Good luck with everything! The whelping pudding is great. I gave it to Tenley (in liquid form, not with the gelatin because she didn't like it that way) during whelping and still give it to her when she gets picky with her food. Also, Red Raspberry Leaf Syrup from revivalanimal.com = AMAZING. I gave it to Tenley for a week before she had puppies, during whelping and then for a couple days after, just to make sure everything was cleaned out. She whelped so easily, and I really think part of it was the raspberry.
Can't wait to hear how things go!