Requirements for using one power supply for multiple sensors
Power is a combination of voltage and current.
For choosing the voltage specification of power supply, find which component requires the maximum power by calculating their operating voltage and current consumed by them, take that voltage.
Since both the servos and the Raspberry Pi work at 5V, go with 5V supply.
Now, since you have fixed the voltage parameter, you need to choose the current specification.
Calculate the total power consumed i.e. the power of servos (5*current) and power of Raspberry pi (5*2.5=12.5W).
Considering high efficiency of power converters, divide the power you calculated by the voltage you just calculated to find current (Power/Voltage=Current).
Once you have calculated the current, assume your power supply will be 80% efficient. Calculate actual current needed using Current/0.8, since actual current*efficiency=output current.
Thus, you need a voltage of 5V and current of 3.3/0.8=4.125 close to 5A (assuming 100mA current consumption of each servo).