According to this 1973 New York Times article it was 50 pizzas and they were sent to the hotel then-Maine Sen. Edmund Muskie was staying at. They were probably not from Domino's, which was just getting started in 1972.I think Segretti ordered 1,000 Dominoes pizzas sent to the DNC as one of his pranks.
Dirty Tricks - The New York Times (nytimes.com)