We do a periodic check on sites to make sure that the moderation team is sufficient for the size of the community and volume of flags. Each site could theoretically get by with only two moderators, we appoint three just so we're certain that the site is covered as moderators go in and out of activity.
Moderation is strictly a volunteer effort, and we're very respectful of the time that our volunteers have to give. When someone has been absent for several months without notifying us or the other moderators that they'll be away for a protracted amount of time, we'll usually reach out to the moderator primarily to see if they're okay.
As we do our periodic checks, we may suggest to the team that we bring another moderator on board, and the moderation team is always welcome to reach out to us to ask the same.
That said, it looks like we've got one moderator on a bit of a hiatus (I'm not worried, I'm quite certain he'd reach out to us if he could not continue, but I will check in to see if all is well), and another moderator stepping down. Hence, I'll be contacting some of the more prolific users that have demonstrated the qualities we look for in moderators, and appointing at least one more by the end of the week.
(If you're interested in becoming a mod, leave a comment to let me know)