Well, we're in the primary phase of the election process, and I looked up at the positions available. I did a double take upon seeing that there were but two positions open. Now, every SE site that I've seen (as far as I can remember) has had three moderators. So then, why are there only two positions available? Are we down-sizing along with the rest of the economy?
The newly elected moderators are not going to replace the existing ones; they are added to them. This means we will have five moderators, after this election.
The number of available positions depends on the needs of the site; if a site requires more moderators, then more positions are available.
As far as I have seen, the minimum number of available positions is one. I have never seen this happen, but it could also be that an election is not held, if there is no need for new moderators.