close all the instances of google chrome except first
To close all windows except the frontmost window, run:

osascript -e 'tell app "Google Chrome" to close (windows 2 thru -1)'


To close all windows except the window opened first, run a script like this in AppleScript Editor:

tell application "Google Chrome"
    if number of windows &lt; 2 then return
    set min to id of window 1
    repeat with w in windows 2 thru -1
        if id of w &lt; min then set min to id of w
    end repeat
    close (windows where id is not min)
end tell


"Instances" are usually called windows in OS X, and windows are not separate instances of a process like the instances opened by open -n.