From http://americangut.org/our-results-so-far/ - "The American Gut project has many more samples representing more groups of people than other studies, such as the Human Microbiome Project, Global Gut, or Personal Genome Project."

No hard numbers on their site, but there's over 7500 contributors on their sample collection kit fundraising page.

They quantify the difference in samples here:

                    HMP  GG  PGP AGP
     Total Samples 4,788 531 683 4,658

http://americangut.org/wp-content/uploads/2016/02/mod1_main....

Thanks!