A Raspberry Pi's GPIO header includes power pins, and enough I/O pins to drive multiple servo motors.
When using this header's 5V supply to power servo motors directly (rather than using external power) it's very easy to place too much load on the PI, causing low-voltage conditions, freezes and restarts. (The most reliable way to power servos is indeed with a separate power supply.)
However for many users, especially with limited resources, the convenience and simplicity of powering and driving the servos directly from the Pi is huge.
What motors can typically be driven in this way before running into problems?