It's either a ton of people and they don't want to admit it OR it's very, very few people and they don't want to be asked the obvious follow up question of "who are they?"
If you're the CIA and you're tracking the cell phones of say 10,000 foreign suspects for 10 years, and each of those phones is making 10 calls/week, you're at 50 million calls to analyze. You could probably look at the country code to get an estimate, but if the other side of the call is a foreign national in the US, they shouldn't count. If the non-US country codes happen to be owned by US citizens who may be traveling, how would you know?
Phones at least have country codes.. How in the world can you assign a country of origin to random email data or plaintext data through a router somewhere?
How much data is stored on each person? How many hoops does an individual analyst have to jump through, if any, to access that information for an individual? Is it stored in an encrypted or anonymized way so that an analyst can't access the details without approval from management? Are there jobs constantly scanning this dataset and alerting on suspicious patterns, and if so, what is the average false positive rate and does an alert give carte blanche to read all of the data collected about that individual and their connections and connections-to-connections?
I doubt Congress will ever get clear or entirely truthful answers to those questions, let alone the general public.