How many bones does an adult human have, roughly?

About 206, and babies start with roughly 270. The count goes down because bones fuse: the skull plates knit together, the sacrum forms from five separate vertebrae, and the three parts of the hip bone merge. Infants have more bones for the same reason a newborn skull is soft - a rigid skeleton cannot be born.

Roughly is the right word for another reason. The number varies between people. Around one in ten has an extra lumbar vertebra or an additional pair of cervical ribs, sesamoid bones form in tendons in numbers that differ from person to person, and a fabella in the knee is present in some people and not others - and has become markedly more common over the last century for reasons that are not understood.

Bone is not the inert scaffolding it looks like in a museum. It is living tissue with a blood supply, continually dismantled and rebuilt, and it remodels along the lines of the loads it carries - which is why a tennis player's serving arm is measurably denser, and why astronauts lose bone mass rapidly in orbit where there is no load at all. The whole adult skeleton is replaced roughly every ten years.