linux - How many DNS Servers must I have?

I am trying to build my own DNS. I have one server which I would like to use as DNS server.

1. Why, according to RFC2182 section 5 (https://tools.ietf.org/html/rfc2182#section-5) I must have TWO servers (3 recommended)?

I tried creating ns1.example.com and ns2.example.com pointing to same IP and it works for some TLDs fine but for my .us domain it doesn't open the website.

2. Why some TLDs does not work with just one server (two ns pointing to same IP)?

Sorry if the quiestion is boring. Thanks!