The acceptance rate of the ISJ is 10 to 12% of submitted papers, based on analysis over the last few years.
However, calculating acceptance rates is not an exact science. The above rate is the proportion of papers accepted for publication in relation to total submissions but as described in the Reviewing Process some papers are rejected before entering the formal review process, for scope or quality reasons. If these are removed from the calculation the acceptance rate is somewhat higher.
If you are considering submitting a paper to ISJ please also see Submitting a Paper