Can I connect a Raspberry Pi and a servo shield to a single power supply?
Read the fine#§* manual RTFM
5v 10A switching power supply (http://adafru.it/658) (up to perhaps 16 servos).  V
4xAA Battery Holder (http://adafru.it/830) - 6v with Alkaline cells.     
4.8v with NiMH rechargeable cells, portable!    
4.8 or 6v Rechargeable RC battery packs from a hobby store.

https://cdn-learn.adafruit.com/downloads/pdf/adafruit-16-channel-pwm-slash-servo-shield.pdf?timestamp=1598199855
Each servo under max load draws 1A . Estimate your Ah or Wh needs and dont undersize it.
It you overvoltage with 2S3P Li Ion you must control acceleration not just position to avoid over heating servos but that might work. A battery balancer extends life.