In the D.A.CH. region (Germany, Austria, Switzerland), arc42 is used by many organizations and corporations in various industries. We don’t know any statistically relevant numbers, though…
In these countries, alternatives like Simon Browns’ SA4D/C4 model are not well-established.
In the international scene, we guess that arc42 is way behind SA4D/C4.
The template is available in English, German and Spanish. Further translations are most welcome (but will not be provided by the current maintainers of arc42).