Here the 1830 typesetter replaced the plural their heads with the singular the head. He may have been influenced by the use of the singular its body in the previous clause (“yea and every limb and joint shall be restored to its body”). Perhaps he reasoned that a single hair could not be assigned to more than one head. Yet he did not make this change earlier in the text, in Alma 11:44, where it reads “and even there shall not so much as a hair of their heads be lost”. In that case, there was no preceding clause in the passage to emphasize the singular number. Of course, there is really no problem at all with this usage in Alma 11:44 or in the original text for Alma 40:23. The meaning is that not a hair from any of their heads will be lost.
Summary: Restore the original text for Alma 40:23 (the reading of both manuscripts), which assigns a single hair to their heads; this same usage is found in Alma 11:44.