Capacitor placement / count
I would suggest putting one capacitor for each servo and place it as close to the servo as you can. You can try different values because it depends on the application. The electronic components inside the servo will produce noise (or glitches) on the VCC line while the servo is operating, so you should place a capacitor (very close) to create a short way to GND for those noise. If you put only one capacitor for all the servos, the noise produced for one servo could reach other servo (or other components on your board) before being grounded.

Sometimes, it is possible that you need to put the capacitor on the servo wires (next to the servo). In my opinion, it would be the best.