I'm using Ubuntu, and I found if I unplug, and then reconnect the usb cable from the computer too fast, the ttyACM number increases. So it appears to me from my experience, you have unplugged and replugged it too fast three times(?). Try unplugging it for a minute, then plug it in again.
You can check the /dev/ directory and see what shows up there when you plug in the Arduino. It should add one port, like /dev/ttyACM0