Samsung Pay is loaded onto the watch independently to the phone and the debit card is separately registred directly onto the watch. This allows it work with other brands and also independent of the phone. There is no way of reporting how many watches are out there on Monzo customers wrists like there is a way of knowing how many phones there is data from installs of the Monzo smartphone app.
Monzo can’t get true figures for this without a survey.
A stop-gap is Curve but it is just a workaround.