The exact number of bones in a human fetus is around 300 total. It must be understood that they are not bones as would aptly describe skeletal support that can be fractured or broken. Rather they are a smooth structure made of cartilage designed to aid passage through the birth canal, as ossification (hardening) only occurs as the child grows older outside the womb. This is similar to how the human skull has fractures in it that gradually fuse together until adolescence. This process also fuses these structures into bones, as they are replaced through calcium growth via protein transcription.
This results in some bones becoming combined into one, which gives us the standard human bone number of 206 in a fully grown adult. This process is not only to replace the modification only designed for birth canal passage, it is because bones need to be hardened with calcium in order to support body weight, a process which human evolution has fine-tuned to this day.