Recently I found a clue, at least for FSA/NRCS/RD, thanks to the requirement for public notice on data requirements. (The first time in my life I've really seen a value for that procedural requirement.)
Here is the Federal Register document from USDA on the information collection requirement for e-Auth.
"The USDA eAuthentication Service provides public and government businesses single sign-on capability for USDA applications, management of user credentials, and verification of identify, authorization, and electronic signatures. USDA eAuthentication obtains customer information through an electronic self-registration process provided through the eAuthentication Web site. The voluntary online self-registration process applies to USDA Agency customers, as well as employees who request access to protected USDA web applications and services via the Internet. Users can register directly from the eAuthentication Web site located at www.eauth.egov.usda.gov. The information collected through the online self-registration process will be used to provide an eAuthentication account that will enable the electronic authentication of users. The users will then have access to authorized resources without needing to reauthenticate within the context of a single Internet session."
"Description of Respondents: Farms; Individuals or Households; Business or other for-profit; Not-for-profit institutions; Federal government; State, Local or Tribal Government.There's no breakdown given for how many of the respondents are actually farmers. My guess would be about 80,000 to 100,000, which might be from 10 to 25 percent of potential users.
Number of Respondents: 114,256.