Based on data from CDC study of American women's contraceptive status from 2011-2013.
How many women use natural family planning?
Not very many!
The National Study of Family Growth, a study run by the CDC, surveyed 5,601 women in the 2011-13 study, to find what family planning methods women were using. Women were asked to answer a long survey (average completion time was 60+ minutes) including whether they "used rhythm or safe period by calendar to prevent pregnancy?" or "Natural family planning or safe period by temperature or cervical mucus test to prevent pregnancy?". Analysis lead to an estimate that only 0.1% (1 out of thousand) of women age 15-44 were using a symptom based method of NFP. Surprisingly, seven times more women (0.7% or 7 out of a thousand) used a calendar based method (the rhythm method). However, even including the rhythm method, less than 1% of women used natural family planning.
A complicating factor in this study is that women were categorized by the most effective method they used - so it may be the case that women using fertility awareness methods in conjunction with condoms were counted as condom users. Additionally, women were not asked if they used breastfeeding to space children, which is often used in conjuntion with NFP.