Suppose I have eight Batman figurines, posters, and other merchandise around my room. Do I have ‘eight Batmans’ or ‘eight Batmen’?
Of course, since the plural of ‘man’ is ‘men’, so it’s natural to assume that any compound noun ending with ‘man’ would have a pluralized ‘men’ ending. However, things get a little trickier with proper nouns. ‘Batman’ is not simply a compound noun; it’s a name. The rules for irregular plurals do not apply to unique proper nouns, accept perhaps for fricative changes to make pronunciation easier (it’s still acceptable to change f to v before adding the es). Changing ‘man’ to ‘men’ and other more significant irregular pluralizations don’t apply to proper nouns.
Therefore, ‘eight Batmans’ is correct, not ‘eight Batmen’.
If you own a restaurant called The Hungry Mouse, then you start expanding to a couple more locations, you now have three Hungry Mouses, not Hungry Mice.
Let’s consider an example you are more likely to encounter, and perhaps have already encountered. Think about some last names you know that end with words that would normally have irregular plural forms. In the U.S. at least, we might meet someone with the last name Chapman, Goldman, Fairchild, Deer, or Silverman, among others. If I join my friend Katy Fairchild for dinner with her family, I go to visit the Fairchilds, not the Fairchildren.
Thankfully, we don’t encounter this issue all that often, but in case you were ever wondering, now you know!