To quote part of R. Hershel Schachter's article on the topic:
It is now several years into their marriage, and our couple has already been blessed with a son and a daughter. What now? The Talmud tells us in the name of Rabbi Yehoshua that even after one has fulfilled the biblical obligation of Pru U'rvu, he is still required to continue to have children in his later years. This idea is derived from the words of Koheles (11:6) בבוקר זרע את זרעך ולערב אל תנח ידך, v'loerev al tanach Yodecho - 'In the morning you should plant your seed and in the evening, as well, you should continue to do the same." The consensus among the Poskim is that this law of Rabbi Yehoshua is not biblical in nature, but only rabbinic.
According to the Aruch HaShulchon, the Rambam's view is that לערב אל תנח ידך does not constitute an independent rabbinic mitzvah, but is rather a Hiddur Mitzvah Min Hamuvchar (a very desirable enhancement) of Piryah V'rivyah. Hence it follows, as the Ramban has pointed out, that although one who violates any rabbinic law is considered wicked (a Rosho) and may be referred to by other people as such, one who refuses to observe this ruling of Rabbi Yehoshua regarding Lo'erev al tanach yodecha would not be considered a sinner. And although Beth Din could force someone to get married even if he did not want to, Beth Din would not force one to observe this mitzvah of having more children than the minimal two. This principle of Rabbi Yeshoshua is a statement of the proper mode of behavior (derech eretz) rather than an official rabbinic enachnent (takkanah).