As of my knowledge cutoff in September 2021, Macau had a total of 47 casinos. However, please note that the situation may have changed since then, and it's always a good idea to verify the information with up-to-date sources.
Regarding the largest casino in Macau at that time, it was the Venetian Macao. It is a massive integrated resort and casino located in the Cotai Strip area of Macau. The Venetian Macao is renowned for its luxurious amenities, expansive gaming area, and a vast array of entertainment options. It features a gaming floor that spans over 550,000 square feet (51,097 square meters) and offers a wide range of casino games and entertainment facilities.