Multiple wireless chargers in parallel or in series and what voltage/current
Figure 1. Source: Adafruit.


  This is a basic charger set, and it does work, providing 5V DC output from the output half when the input half is powered with 9V to 12VDC.


You connect these devices in parallel in the same manner that all the devices in your home are connected in parallel to the mains supply or all the devices in your car are connected in parallel to your battery.


  Should I hook these up in parallel or in series, and at what power outputs?


Parallel. The power required for the driver side is the sum of the powers of each device.


  Also if I have all the circuits hooked up but only have one device charging out of the 10, would this effect it?


The advantage of parallel connection to a constant voltage power source (mains in your house, 12 V supply in your car, 9 V supply for these devices) is that each device will draw the current it requires to operate. Provided the supply is capable of supplying enough current to power the maximum simultaneous load you will not have any problems. Exceeding power supply rating will cause the supply voltage to droop and at a certain point the devices will fail to operate properly.