I am willing to bet it is almost zero. Android does the job for most people that crave modding-ability/customisation, so Ubuntu Touch is really only interesting for a very small niche.
Among the people i know who really care about open source and security, the max is running Android without the Google Services, but i have never seen Ubuntu Touch in the wild.
The problem is, even most apps from Google's competitors now won't run without Google Play Services. Android is becoming increasingly useless as an open source OS.
Among the people i know who really care about open source and security, the max is running Android without the Google Services, but i have never seen Ubuntu Touch in the wild.