Would you use, two BlackBerries? Or two BlackBerrys?
Today with so many divergent proper names it is easy to pick the usuage from the ones most commonly used. For instance. We see that the plural used for the an Emmy is Emmys. The same holds true for Grammys and Tonys.
We can also look at the car industry for some clues. Using the Camry you notice they pluralize it as Camrys. Therefore, two BlackBerrys is correct. Although to the eye it may seem a bit odd at times.
Now, after following the current trends to popular proper names. You should fall back to the tried and true of adding an 's' or 'es' for Personal Names, Nations, and Nationalities.
Lets take a quick look at each one of these. Then as you write you will not have to pause and spend time digging up the correct answer, before submitting your next project.
The first names of personal names are pretty straight forward. If you have four girls named Betty. You simply write, four Bettys in fifth grade. You will notice however, in a lot of spell checkers it will highlight or underline these spellings as wrong. This is why I'm writing some of these tips out. It is easier to learn them now, than to have publishers sending back your work because you missed them.
Surnames, or last names as we call them today, are a bit different. If the surname ends in consonant or a vowel. You treat it the same. You add an 's' or 'es'. For example. Johnson becomes Johnsons. Corleone becomes Corleones.
Names that end in 'es' or 'ez' you add 'es' to the end of them. This way Jones becomes Joneses. And Chavez become Chevezes. Again, to the eye it may seem odd. But proper usage in your writing will aid you in getting published faster.
What about Nations and Nationalities? Adding an 's' to Nations names is the usual way to write them. There for Hungary become Hungarys. If however, the name ends in an 'I', get an 's'. Which is why we see for Israeli, becomes Israelis.
Now when writing about Native American Tribes you have a choice. If you are writing to be published by a publisher. You should defer to the Chicago Manual of Style. They use an 's' as the plural form. Thus Mohawk become Mohawks. Hopi becomes Hopis. And Cherokee becomes Cherokees.
If you are self-publishing you can use the Merriam-Webster's Collegiate Dictionary and use the proper name as the plural itself. For example, Hopi is both a single Hopi Indian, and the Hopi nation as well.
On Writing Well, 30th Anniversary Edition: The Classic Guide to Writing Nonfiction