I would suggest you just do whatever makes it easier for people trying to interpret the play...
So in my opinion, no, there's no need to mention every individual character by name. You're best off giving them a group name like "MONKS" if they share the same line.
An example of something similar would be in Romeo and Juliet, where Shakespeare uses a "Chorus" to open the play. This is a group of people, but because their individual identities don't matter, they aren't assigned individual names.
If, later in the play you decide to have individual monks speaking a few lines, you could use MONK 1 or MONK 2.
However, if they are going to develop into a full character (no matter how small the part), then a name would be a sensible option.