What is the plural form of trademarked product names, specifically of the term "WordPress"?
Solution 1:
Whether there is a plural form depends entirely on whether there is actually a singular form.
In the case of WordPress, there isn't a singular form. You don't say “I implemented my blog as a WordPress.” It’s using WordPress or even on WordPress or in WordPress, but not as a WordPress.
Consequently there is no plural form.
This doesn't apply to all trademarked names, though. One may very well talk about Compaqs or Pepsis or Hoovers or even Guinnesses and Tumblrs. All of these have a singular form — a Compaq [computer]; a [drink made by] Pepsi; a [vacuum cleaner made by] Hoover; a [proprietary drink made by] Guinness; a [blog built on] Tumblr.
Additionally, it probably depends to some extent on how euphonious the plural form (if there might be one) actually is. Even though one might talk of a Kleenex for a tissue, a few Kleenexes is unlikely to occur. WordPresses may well fall into that category as well as the “no plural” category, even if Guinnesses does not.
Solution 2:
I think there is a fundamental misconception here about the “rules” of English (or any other language). They aren’t regulations; they’re rules-of-thumb which tell you “do it this way and you are unlikely to be misunderstood.”
There are rules which tell you how to construct a regular plural of a singular noun. By analogy with other words whose singular ends in -ess, the plural of WordPress would be WordPresses. If you write WordPresses the “natural” (meaning “conventional”) parsing would understand this to be the plural of WordPress.
That rule, however, does not prohibit you from forming the plural otherwise. You may, if you like, write of three WordPress or three WordPressi or three WordPrexx. If your plural is understood by your readers, and is taken up by your readers, used by them, becomes the standard, the regular-plural-in-es rule will not smack you down. It will simply shrug and move on. You and your readers will have written a new rule, which is exactly as valid as any other rule in the rulebook: the plural of WordPress is WordPressi, or whatever you all use.
Likewise, that rule does not tell you what the plural of WordPress means. That’s a different set of rules. The analogy with Coke and Toyota gives a rule that WordPresses is likely to be understood if used to designate units of purchase or consumption. The analogy of Hamlet and Frankenstein gives a rule that WordPresses is likely to be understood if used to designate distinct editions or versions.
But those rules do not impose those meanings, or preclude others. That’s up to you and your fellow users. If you choose to use WordPresses to designate multiple purchases, or multiple versions, or multiple installations, or anything else where you feel a plural is called for, then whichever of these uses your readers understand and adopt in their own use becomes a meaning of WordPresses. You and your readers will have written a new rule, which is exactly as valid as any other rule in the rulebook.
When you do so you will undoubtedly encounter opposition from people who don’t understand how languages work—who sniff and whimper that you’re “bastardizing the English language”. That’s fine. They’re users, too; they get a vote, just like you.
I don’t get a vote now; but if tomorrow I decide to start a blog using WordPress, and I come over to wordpress.SE and start reading and posting, I will have become a citizen of that particular linguistic community and I will get a vote, too.
The WordPress Foundation may have something to say, and they get lots of votes, because they publish so much and have so many readers.
ELU, however, doesn’t get a vote. Merriam-Webster doesn’t get a vote. The OED doesn’t get a vote. None of these is a user; they just report the election results.
Solution 3:
The term WordPress is a trademark; as such, I view it not as a noun but rather as an adjective, and therefore neither singular nor plural. It is a proper name that describes a software script.
The use of the term as an adjective also applies to the example: "I have two WordPress installations," or, "my server runs two WordPress instances."
The use of the term as an adjective also fits well with the cola analogy, and the trademarks Pepsi and Coke: "I would like two Pepsi colas, please," or, "I would like to Coke drinks, please." In these cases, Pepsi and Coke are adjectives that describe a particular beverage.
In the case of Coke, Pepsi, and Toyota, the trademark is being commoditized or genericized, such that the trademark becomes synonymous with a good or service. (For example: we all use kleenex rather than facial tissue; residents of Atlanta order cokes, not sodas; and Brittish housekeepers hoover, rather than vacuum, the carpet.)
In such cases, the trademark becomes a synonym of the noun that it formerly described (or the verb that defines the action performed by that noun, such as hoovering) - usually accompanied by referring to the term in lowercase, rather than capitalized (Kleenex/kleenex, Coke/coke, Hoover/hoover).
As popular as WordPress is, it has not reached the point of being commoditized; as such, I would continue to treat it as an adjective.
That said, a non-trivial segment of users - likely, comprised of the same people who refer to having a facebook, or a twitter, or a tumblr - view WordPress as a noun, and refer to having a wordpress. In such cases, I would consider the wordpress to be the hosted, served, running script; i.e. not merely the software script itself, but the resultant application comprised of the WordPress software script, the LAMP stack, the server hosting it all, and the browser rendering the output. At that point, I would consider that application to be commoditized - similarly with Facebook, Twitter, Tumblr, etc. - to the point of being synonymous with the application itself. Thus, in such cases, I would consider "WordPress" to be a noun, for which the proper plural would be WordPresses.
And as someone who still views WordPress as the trademarked name of a software script, that makes me cringe even more than hearing someone talk about their "facebook" or their "twitter".