How much is the teaching duty of university administrators? Due to the time-consuming nature of administrative jobs, university administrators normally reduce their teaching duties. This reduction is normally proportional to the level of administrative job: Chair/Dean/Vice President/Provost/President.

I am curious if it is only matter of personal preference or there is institutional regulations for minimum and maximum teaching tasks?

In general, how university administrators are involved in classroom teaching, supervising graduate students, and academic research projects?
You are right that professors in various administrative positions typically have a reduced teaching load. However, this is hard to quantify and depends mostly on the institution's policy and maybe even the specific agreement (or contract) negotiated. The full range is possible: from no reduction in teaching load (position with little time requirement) to full “exemption” from teaching (e.g., dean or university president).