Hi. We don't know for sure how many targets are involved in these letters. However, the USA Freedom Act prohibited bulk collection of information here.

The government now releases basic statistics on NSLs, for example here: https://www.dni.gov/files/CLPT/documents/2019_ASTR_for_CY201....

In 2018, there were 10,235 NSLs, with 38,872 "requests for information" (ie targets) within those NSLs. So you can make some general inferences.