Do multiple ultrasonic sensors need an external power source?
Well, according to the datasheet the "working current" is 15mA.  Even if you had all 7 working at once, which I doubt you will - you'll be working through them in sequence I guess - that would be just 105mA - way below the 800mA the Arduino's on-board regulator can supply, and only a fifth of what a USB port can provide.

The datasheet doesn't mention the quiescent (idle) current, but it will be a tiny fraction of that 15mA and so can mostly be ignored.  Running the sensors in sequence, which is the most likely scenario, would use about the same amount of current as if you had 1 sensor - or 20 sensors.  Most of the current consumption would be while you are actively producing an ultrasonic chirp.

So no, you don't need an external power supply to run them all - just take them all from the Arduino's 5V pin.