This question is not as straightforward as it might first appear. For a start, it depends exactly what we mean by the question. If we are asking how many elders do we need for our church to be considered legitimate, the short answer is none. Let me explain.
In the New Testament, it is apparent churches existed before there were elders. The Apostle Paul repeatedly refers to churches as churches even when those churches have no elders in them. He famously told Titus to appoint elders in the churches. That is, he calls them churches. They are recognised as churches. That is before any elders have been appointed because Titus is dispatched to help them do just that. The same is true throughout the book of Acts.
Local churches existed as legitimate local churches before there were any elders in any of them. Now, they may not be healthy churches. That may not be the long-term goal. It may not be the design Jesus wants us to aim for, but it is nevertheless the case. Churches existed before there were any elders in them. So, if by our questions we mean something like this, ‘how many elders do I need for my church to legitimately be a church?’ The answer has to be none.
However, if our questions is something close to ‘how many elders does God want us to have?’ I think we have an altogether different question on our hands. A question to which there are two biblical answers. The first and most straightforward of which is this: more than one.
You cannot escape that elders, wherever you find them in scripture, are plural. Every example of elders being mentioned in the New Testament comes in plural, except three instances. But these three singular examples refer either to one elder writing a particular letter (cf. 2 John 1:1, 3 John 1:1) or concerns an accusation against a specific elder (1 Timothy 5:9). But every mention of every church either appointing elders or with reference to its elders refers, in every instance otherwise, to a plurality of elders. The pattern of scripture for local churches is clearly and evidently a plurality of elders. This means, in the ordinary run of things, God intends for local churches to appoint more than one elder.
However, though the pattern is multiple elders in individual churches, there is nothing directly commanded about the number of elders to appoint. This makes some sense. Clearly in the planting of new churches, that are biblically legitimate, it will take some time before elders can be appointed. I would argue that the appointment of elders is a matter of recognising the gifts that God has given to the church (a post I might flesh out another time). It is for the church to recognise qualified elders, but it is ultimately the Lord himself who gifts elders to the church. Given that local churches legitimately existed without elders, and the church is called to recognise the men gifted to them by God, the pattern of multiple elders in local churches is the biblical model God intends us to follow and towards which we ought to work. Which leads to our second, more nuanced, answer to our question. How many elders does God want us to appoint? It depends.
We ought to be working towards more than one. However, we can only appoint whomever God has gifted to the church. If he has not gifted a church with anybody suitable then, such as they are a legitimate church, they must be so without elders. They ought to aspire to a plurality. Biblically, they should be working towards that end. But for the time being, they have no elders to appoint so must accept that is the Lord’s provision for them at this time.
As I note in my forthcoming book (due to be released later this year) and where I flesh this out in more detail:
What we are aiming for is a plurality of elders, but we recognise we can only appoint whomever God has gifted to us. We cannot put a ratio on God’s grace or demand that he gives us what he, in his sovereignty, has determined not to grant. In the end, it is better to have no elders than unfit ones. But as a matter of principle, it is God’s design for individual local churches to have multiple elders and this is what we at least ought to be working towards.