This is basically a wording issue because the requirements in the tutorial are not a general statement about replica sets, rather they pertain to the tutorial itelf. The requirement for deploying a replica set in general is 1 or more, most have 2 or more, but the tutorial you link to is specifically to deploy a 3 member set:
This tutorial describes how to create a three member replica set from
three existing instances of MongoDB
Hence, the requirement for the tutorial is that the set have 3 members. Nevertheless, it is a little confusing to have the two statements, so I submitted a pull request (EDIT: pull request has now been merged in) to clean it up a bit.
You can have a single mongod node configured to be part of a replica set, but strictly speaking that would not be a "set". In fact, since replication uses more resources than a standalone
mongod (basically because of the oplog) there is really no point to running a single member unless it is as a temporary measure before you add other members to the set.