What exactly do the range of numbers (1 to 7) mean on the temperature knob/dial on a basic window air conditioning unit?
The numbers are completely arbitrary, and are whatever the product manager or artist at the manufacturer decided they should be.  They might be standardized within a company's product line.  If these are private-label units, all bets are off. 

Numbers instead of real degrees means they used a thermostat too cheap to be consistent from unit to unit, so they couldn't print degrees and have that be meaningful.  

Alternate explanation: they market the identical unit in several countries, and one of them uses Fahrenheit, so rather than have 2 scales, they have none.