grammatical number - What is the plural form of trademarked product names, specifically of the term "WordPress"?

On the stackexchange site WordPress Answers, we recently discussed the plural form, or whether one exists at all, of the system we all use.

WordPress is a free and open source blogging tool and a content management system (CMS). In more colloquial terms it is a software application or "program".

Initially, I suggested that analogously to "press" and its two-word-companions such as "bench press" the plural ought to be "WordPresses".

Another user argued, that it was a singular without a plural, since it is a trademarked name of the WordPress Foundation and hence any alteration of it would be a fork (programming terminology).
The latter is obviously for one correct and for another in the realm of our expertise and not to be discussed here.

Still, initially I contested the earlier, since I thought I could for instance order "Two Pepsis" or "Two Cokes" (synonymical to "Two bottles/cans of [...]") in a restaurant and inline with that "WordPresses" would not have to mean "the system and a smiliar one (i.e. a fork)" but could refer to "several installations of WordPress".
A third user added that that might be misused in spoken language frequently, but that it would be correct to order "Two Pepsi" only and that that term didn't have a plural either. That still felt wrong to me, but thinking about it further, I concluded that you'd never order "Two Guinnesses", but "Two Guinness" instead and that hence that must hold true for Coke and Pepsi as well - and because of that ultimately for the initial term (WordPress) also.

They almost have me convinced, but whatever the correct way, this has sparked my (and likely our) interest and I'd like to get a definitive answer on this one. Hence I am asking you guys and girls here. Shed some light for the IT nerds, please: Do product names have plurals or not? Or, if this can't be answered categorically with yes or no, in which cases do plurals exist and in which do they not?

Answer

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.