Actually no one really knows, and there isn’t a single, simple answer to this question. It is frequently a hotly debated topic between cat and wildlife advocates, because estimates of cat predation starts with community cat population assumptions.
GUESSTIMATES vs. SCIENTIFIC APPROACH
There are estimates cited in peer-review journals and estimates based on surveys; the least accurate should really be called “guesstimates” as they are just estimates based on a multiple of the pet cat population (typically 100%).
The most common number cited in journal articles is 100 million feral cats. But this number tracks back to an estimate originally published by a TNR organization that presumed the number of feral cats was a multiple of the number of pet cats. There is nothing scientific in this approach, and no reason to think it correlates to the actual number of feral cats.
Ann Beall, in Community Cats: A Journey Into the World of Feral Cats (2014) surveyed 1,500 U.S. residents (Internet based survey). Based on survey results multiplied by the number of U.S. households, there are an estimated 69 million stray or feral cats.
The feral cat population estimate closest to something based on traditional wildlife population management methodology belongs to Merritt Clifton of ANIMAL PEOPLE, who estimates that the winter feral cat population may be as low as 13 million and at the summer peak is probably “no more than “ 24 million. The estimates were projected from the typical numbers of cats found in common habitat types taken from a national survey of cat rescuers and cross compared with animal shelter intake data. The notion that there are fewer feral cats than generally cited in the literature (then and now) is supported by road kill data gathered by ANIMAL PEOPLE from a number of cities around the country.