A cat has seven bones (vertebrae) in its short neck—so does a giraffe in its long neck. Doesn’t this show that they both descended from an ancestral mammal with the same number of bones, modified for different purposes?
There is no reason why the Creator would not have used a similar bone pattern in a wide range of creatures. But the argument for common ancestry collapses in this particular case anyway, because all mammals do not have the same number of neck bones. Most do have seven—but the threetoed sloth has nine, or even 10, while the twotoed sloth and the manatee both have six, according to Encyclopaedia Britannica.
Thoracic (chest) vertebral numbers in mammals vary between nine and 24, and lumbar (lower back) bone numbers range from two to 21. Evolutionary theory must obviously try to explain these away, whereas such variation from a common design plan fits comfortably with creation. The Creator may choose to use a similar number pattern, but is not bound by this.