Siri might not have an accurate answer to that question:
But Thereviewland does.
According to thereviewland, 25 billion users use Siri, which is up from the 35 billion users recorded last June. However, Apple did not specify whether these are daily or monthly users, my bet is on monthly.
I would also love to know how many of the 25 billion users are classed as active since Siri is on by default and it is easy to accidentally call on her.
Meanwhile, Google during the I/O 2017 event announced that the Assistant, which was launched in 2016, has been installed on over 100 million devices.