This is her first time, how many do great dane's gives birth for on the first time ?

Oh, maybe three to fifteen. You and she may be pretty busy ! Congratulations !

The size of any litter can vary, it just depends on how fertile both male and female are and how many times she's been bred.

You should become close friends with your vet... You can have an ultrasound done any time now and get a good estimate. Around 43 days, X-rays should be taken to count skulls and spines and to see if the puppies can pass through the birth canal.

Dogs pregnancies are about 60-63 days. Purchase books on what to do and how to set up a welping pen.

I also hope that you have about $2000 stashed away for any emergency. Large breed dogs are not known to have difficult births, but it does happen and emergency c-sections should be added into this ordeal.