How many nameservers do I need per domain

Two. Registries require at least two nameservers for a domain, and for almost every estate two is also the right number to stop at.

Why two is the minimum

A single nameserver is a single point of failure for the whole domain. If it does not answer, the domain does not resolve, and nothing about the site being up matters because nobody can find it. Registries refuse single nameserver delegations for exactly this reason.

Why more than two rarely helps

A third or fourth nameserver adds redundancy that is already covered, and it adds records to maintain. Resolvers query one nameserver at a time and fall back on failure, so two working ones give you the resilience; the fifth changes nothing a visitor would notice.

The exception is genuine geographic distribution across separate infrastructure, which is a different exercise from adding names in the same place.

Private nameservers do not change the count

Running ns1.yourdomain.com and ns2.yourdomain.com instead of shared ones is about what a lookup reveals, not about resilience. You still need two, and they are still two.

What private nameservers do change is the footprint your domains share, which is the whole point on a network and is covered in why shared nameservers undo IP diversity. Creating them is described in creating private nameservers.

One set or one per domain

This is the question that actually matters on an estate, and it is not about the number two.

• One set for everything is simple to run and means every domain in the estate shares a visible common point.
• Several sets across different domains breaks that link, at the cost of more to maintain.

Which is appropriate depends on what you are actually protecting against, and it is worth deciding deliberately rather than by default. The trade off is worked through in planning nameservers for a multi domain estate.

Both must actually answer

The common mistake is delegating to two names where only one resolves, usually because glue records were created for the first and forgotten for the second. The domain works, so nobody notices, until the working one is unavailable and the domain disappears.

Check both by name after any nameserver change, not just that the site loads. Confirming DNS is doing what you think is covered in confirming your DNS.