PCA9685 on PR21-14

Is it recommended to use the +12V on each individual channel or could the same 12V supply be used to supply power to solenoids and then the ground would be connected through the PR21-14 controlled ground?

A single 12V can be used. In the end it depends how much power each solenoid is consuming.
if each solenoid is consuming around 12V 250mA then yes, single power supply will work. but if each is consuming 12V 5A then not.

To clarify, where it is marked +12V on each channel we would connect +12V from our power supply if our solenoids used for example a maximum of 500 mA or is that meant to be a +12V connection for the solenoids?

all 12V connection are shorted together. the power supply can be connected to 12V ( power in screw terminal) and solenoid can connect to the output channels directly