In the gut flora, anaerobic bacteria outnumber the aerobes by a ratio of:

The stomach's acidity keeps the number of microorganisms at a minimum (102-103/mL of contents). In the adult duodenum, there are 103-104 bacteria/mL of effluent; with higher populations in the jejunum, 104-105 bacteria/mL, and ileum, 108 bacteria/mL; and in the cecum and transverse colon, 1011 bacteria/mL. In the sigmoid colon and rectum, the bacteria constitute about 60% of the fecal mass. Anaerobes outnumber facultative organisms by 1000-fold. In diarrhea, the bacterial content may diminish greatly, but in intestinal stasis, the count rises. Ref: Brooks G.F. (2013). Chapter 10. Normal Human Microbiota. In G.F. Brooks (Ed), Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.