The National Security Agency (NSA), which is behind some of the world's most sophisticated mass surveillance operations, can't say how many Americans it's spying on in these endeavors. That's not because it's a secret, though that might be a reason too. It's because the agency's operations are so vast that it can't even figure out the number.

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According to Quartz, National Intelligence Director James Clapper said at a press briefing that the agency has no idea how many American citizens it spies on. "We're looking at several options right now, none of which are optimal," he said. "Many people find that unsatisfactory, but that is a fact."

U.S. senators Ron Wyden, Mark Udall and others asked the agency for a ballpark estimate on April 22nd. "We are not asking you for an exact count. Today, our request is simply for a rough estimate." The NSA couldn't answer.

In theory, the NSA is forbidden from spying on U.S. citizens. But in practice, a secret 2015 court ruling unsealed this week reveals that warrantless spying has been approved by the Foreign Intelligence Surveillance Courts for general investigations in the U.S. Furthermore, the NSA says it wants to share access to communications databases with other domestic law enforcement agencies, including the FBI.

But the reason that the NSA can't answer this important question yet is that its surveillance operations are massive, and data might overlap. There are two major spying programs that collect data about U.S. individuals, revealed by the Snowden leaks as PRISM and Upstream.

With PRISM, the NSA requests data including emails, texts , video chats, photos, and more from tech companies, which have to provide it. Upstream is a goldmine of data, on the other hand. The NSA is believed to tap about 80% of the world's data traffic via undersea fiber-optic cables.