𝓞 is extant here and reads man, as does 𝓟. The 1830 edition replaced the singular man with men, perhaps accidentally. This change is consistent with preceding and following uses of the plural men, and it may have been prompted by them:
In these two instances, the plural is appropriate since Alma is discussing the resurrection of different individuals, who will not necessarily be resurrected at the same time. On the other hand, the singular man in “and time only is measured unto man” works well since this is a general statement about time and mankind.
It should also be pointed out that verse 10 reverts back to the singular man. This instance refers to the times of resurrection for mankind in general, so once more the singular man is used:
We see a similar example later on in Alma’s discourse with Corianton: “but behold it was appointed unto man to die” (Alma 42:6); again we have the meaning ‘mankind’. Thus there is no reason to change the instance of man at the end of Alma 40:8 to men; the critical text will restore the singular man.
There have been quite a few cases in the history of the text where man and men have been mixed up. For some examples, see under 1 Nephi 15:35.
Summary: Restore the singular man at the end of Alma 40:8 (“and time only is measured unto man”), the reading of both manuscripts; the singular is acceptable when the meaning is ‘mankind’.