To he best of our knowledge, it is calculated, based on the publications and citations that appear on a given Google Scholar site. The h-index then gets calculated in this way:
First, publications are ranked in descending order by the number of citations received.
Second, the value, h, is calculated as equal to the number (N) of publications that have N or more citations.
h index is the "N of publications having at least N citations". For example h index is 17 in your case it means your 17 publications have atleast 17 citations. h index is unchanged for more than over a year in your case because citations of only those publications may have increases which already contributed in calculation of h index