How to prevent a voltage drop when a motor turns on?
Use a better power supply than a 9V battery- one that does not change output voltage significantly regardless of the number of motors. You could also add a regulator. 

For example, a 12-volt SLA (Sealed lead-acid) battery with a suitable regulator for the currents involved would be almost perfect. Even without the regulator it would be pretty good (provided your motors were happy enough with the higher voltage). 

The problem you note will only get (much) worse as the battery is depleted and its internal resistance rises.