64 in the official Credenziale.
Enough for a stamp a day if you stay within the limit of the "standard" legs: 45 from Gran San Bernardo to Rome.
And you don't have to get two stamps every day in the last 100 Km.
Moreover, a good news is that if you need more space to collect stamps, you can make an additional "home made" Credenziale. It will be acknowledged and accepted anyway to get your Testimonium in Rome.
I called an Italian hospitalero and he confirmed me that this year the VF Organization is having an unexpected huge demand.
They are reprinting the Credenziale and it will be available again in a couple of weeks.
In Torino, as far as I know, there is nobody who sells it.
If it could be easier for you, I could buy one ASAP and give it to you when you come here.
Credentials can also be obtained from Canterbury Precinct UK . If you send them an email to find out the postal costs and send the a cheque they will send you your credential by post> email@example.com