A minimum of 2 nameservers are recommended for each domain name. This is to provide Load Balancing and promote Redundancy.

Load Balancing

Nameserver requests are equally distributed among all of the nameservers listed on a domain name. By listing additional nameservers you decrease the load of requests each nameserver must process.

Redundancy

Providing additional nameservers will also provide an alternative nameserver if a request to one nameservers fails. That way if one of your nameservers is down your domain name should still resolve correctly through your other nameserver.

This depends on which registry the domain name is registered through.

Some registries, like the .com and .net registry allow you to list only 1 nameserver for your domain name. While other registries, like the .org registry, require that a minimum of 2 nameservers be listed for each active domain name.

No, the nameserver requests are equally spread among all of the nameservers, regardless of the order of the nameservers. No priority is given to the Nameservers.