The simple answer is a lot. The statistics range from 49% of households in a study by the US Census Bureau to 68% in a survey by the American Pet Products Association. This can include up to 90 million dogs and 94 million cats.
Why the differences? Methodology. Some studies use mail and some use the internet. And, studies that target the group such as the American Pet Products Association are more likely to have pet owners respond to their survey than general surveys like the census.
For more information see: https://www.columbian.com/news/2019/feb/22/fuzzy-statistics-how-many-of-us-have-pets/