Faceless masses, as described in the TV Tropes website, is usually what animators resort to when it comes to creating masses of people without needing to go into great detail to depict who those people are, in order to make the story world feel populated.
In both the Smurf comic books and the cartoon show, the identical appearances of most of the Smurfs in general, coupled with the fact that most of the unnamed Smurfs are simply called Smurf, serve this function, with creators of the stories being able to bring in characters from among the 100 Smurfs in the village to become prominent for a while or be the focus character of the story and then easily write them out without having them remain a permanent part of the overall cast. This can serve to eliminate characters that end up becoming unpopular with the fanbase.
However, it should be noted that, due to the limits of cartoon animation at the time the Smurfs cartoon show was produced, the overall number of Smurfs that appear in a collected crowd can seem lower than 100. Sometimes the lower number is justified by the story itself limiting the number of prominent characters in it with some anonymous characters thrown in. This problem was most likely rectified with the use of CGI animation in the 2011 Smurfs movie.