What is the minimum number of candidates needed for an election of moderators to proceed to the final, election, phase? What happens if there are not enough?
It depends on the type of election. For this one, a regular one, if there are not enough candidates to have a competitive election (three, because there are two seats), the nomination period is extended by one week. If after that week, there are still not enough candidates, the election is cancelled.
(The rules are different for pro-tempore elections and graduation elections on beta sites; in those cases, the candidates may simply be appointed in the end.)
For more information, have a look at There's an election going on. What's happening and how does it work? on Meta Stack Exchange.