They didn't act alone, the paid number had to be approved by someone and the phone provider also had to accept the bill. I don't have numbers but since I know multiple people who got the strange phone bill my guess would be at least hundreds of thousands of succesful click thoughts. Some called the tax office regularly around the busiest time of the year before they got the bill and found out.

There are probably people with large phone bills who didn't notice and ones who thought the tax office was just expensive to call.

Google let similar things run in Germany that were impersonating government offices (which is illegal in multiple ways), and they never lifted a finger when the malicious ads were reported, because they still made money in the end.