The objection that Jesus didn't appear to enough people isn't new. It was raised by Celsus in the second century, and Origen considered it something "which is not to be lightly passed over" (Against Celsus, 2:63). The objection has taken different forms. Sometimes it will be asked why Jesus didn't appear to more non-Christians. Other times, it's asked why He didn't appear to more people in general. Or both points might be brought up.
I've addressed this subject in the past, such as here and here and in chapter 11 and appendix 7 of The Infidel Delusion. What I want to do here is provide a summary. You can get more details by reading sources like the ones I just mentioned.
This objection is similar to another one that's popular today. Often, skeptics will ask why Christians don't believe in the miracles attributed to Apollonius of Tyana, Marian apparitions, or something else they present as a parallel to the resurrection of Jesus. We've addressed that objection before, such as here. It's similar to the objection I'm currently addressing in that it doesn't explain the evidence for the resurrection. Rather, it's attempting to demonstrate Christian inconsistency, the alleged insufficiency of the case for the resurrection even if it's true, or something else. Whatever merit such an objection has, it should be distinguished from other objections, which attempt to argue that the resurrection didn't occur. Asking what Christians make of the evidence for another miracle or asking for more evidence for the resurrection doesn't explain the evidence we have for the resurrection.
And we should remember that the evidence for an event can involve more than seeing the event occur. The empty tomb, which was accessible to far more people than those who saw the risen Jesus, was evidence for the resurrection. So was the testimony of those who saw Jesus risen from the dead. You wouldn't have to be in Paul's place as a resurrection witness in order to have access to Paul's testimony. We're confident about thousands of historical conclusions and verdicts in law courts, for example, based on the testimony of people who witnessed things that we didn't witness. It would be beneficial to have more evidence, but we distinguish between sufficient evidence and evidence that's more than sufficient. Should we expect more evidence just because some people demand it or would prefer it?
The New Testament doesn't tell us how many people Jesus appeared to. It mentions a few hundred people, but it doesn't tell us whether others were involved or how many.
It also doesn't tell us how many of them were non-Christians. The guards at the tomb witnessed some of the effects of the resurrection, even if they didn't see the risen Jesus. The language of Matthew 28:11 suggests that there were at least three individuals involved. The accounts of Paul's conversion in Acts tell us that at least two non-Christians were with Paul on the road to Damascus. James seems to have been a non-Christian when Jesus appeared to him. See here. And Jesus may have appeared to at least one of His other brothers who weren't Christians at the time. See my discussion of that possibility in appendix 7 of The Infidel Delusion. The evidence suggests that at least seven individuals mentioned in the New Testament were non-Christian witnesses of the resurrection in some form: the guards at the tomb, James, Paul, and Paul's travel companions.
At least two of them, James and Paul, were prominent leaders in the early church. If the early Christians not only had several non-Christian witnesses they could cite, but even had some of them within the highest ranks of their leadership, critics will have to explain why more was needed.
Given the inadequacy of skeptical theories that attempt to explain the evidence we have for the resurrection, the objection that there isn't more evidence is dubious. What we have is more than sufficient, even though we can imagine ways in which the evidence would exceed what's sufficient by an even wider margin.