I tried to shut out 6 Access garrisons to get the achievement, but either there are only 5 in this Access (bug) or DC leaves a garrison unrevealed (possibly intentional). I shut down all the known ones and didn't get the achievement. Reporting in case this is a bug.
Never mind, I did just get the achievement. So it is just odd that there are only 5 garrisons here when I thought there were supposed to be 6 in Access.
There can be up to 6, but not always :)