Siri might not have an accurate answer to that question:
But Tim Cook does.
According to him, 500 million users use Siri, which is up from the 375 million 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 500 million 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.