"All Platforms" should also include:
In which case I am sure we will see FreeBSD being quite a bit higher. Possibly around 5%.
For server, that's not the case; in practice only Linux and some Windows exist. Obviously, there are quite a few Windows servers used in businesses, and cloud providers (in particular Microsoft Azure, duh) have them available. But the vast bulk of servers are Linux, in particular in two groups that use the vast majority of all servers in the world: Supercomputers (where Linux has 100% marketshare of the 500 largest ones), and the cloud super scalers (FAANG). The only large cloud company that uses any FreeBSD is Netflix, and rumor has it (the Netflix HQ is in the same town I live in, and we know people) that they have been replaced by Linux. I would be surprised if even 1% of all servers worldwide run an OS other than Linux and Windows.
For embedded, that's a really tough question, because many embedded systems do not interact with computer networks, and if they do, only in a limited fashion. For example, my wall thermostat and my WiFi AP all run Linux; the garden sprinkler controller runs Android. But the AP never sends any packets to an outside network, except to its own manufacturer to get firmware updates. And the thermostat and sprinkler speak to their cloud service, and to nobody else (yes, I monitor that occasionally). It is possible that my dishwasher or refrigerator run FreeBSD internally, but how would I ever find out? They have no screen, the only output device is a few indicator LEDs, the only input device a few buttons. It could be that FreeBSD has a 5% market share in this type of application, but why does that matter to the typical user here on the forum, who uses FreeBSD either as a server or as a desktop?
Aren't these things related? More users = more developers.
On the contrary. The four most commonly used OSes for computers with user interfaces are (in that order) Android, iOS, Windows and MacOS. While each of them has a very large number of developers (I bet each of them has many thousand coders), their users are completely not involved in developing. They can't even file bug reports in the usual sense of the word (where you get feedback about the bug), nor can they see OS-level documentation (except for Android, which is partly open source, but doesn't accept much input from outside).
In the specific case of FreeBSD (or Open- or NetBSD): If these OSes got a million new serious desktop users, I think what would happen would be the opposite of having developers have more time for doing development. I think the clueless segment of the user population would clog channels such as PRs and developer mailing lists. Helpful developers (BTDT) would waste their time on helping users; the rest would probably go into isolation or leave the project.
If you want to distribute software to customers or consumers, you have a choice. If you want to provide technical support, you need to create a technical support organization and infrastructure which is capable of handling the workload, including the fact that half of the users have an IQ that's below the median for the group. This is the model used by expensive commercial software: People who spend 5- and 6-digit amounts on software expect their phone calls and e-mails to be answered, and if the question is difficult, they expect the person responding to have a doctoral degree. This model sort of works for cheap consumer software: If you have problems with iOS or MacOS you can walk into an Apple store and get limited help, Intuit and Microsoft at least theoretically have support systems (mostly web-based and automated, and usually of not much help). For free software, that model doesn't work, because the money just isn't there (duh). The other support model is to offer no support. That means support questions get moved to a different place, for example this forum. It also implies that some (charitable and laudable) developers help out with support. But speaking as the manager of these developers: they do have more important things to do. This is the system free Linux offerings use (if you buy RedHat or SUSE, you actually get support, that's where most of your money goes).
The vast majority of users of free software do not turn into developers, nor do they help with developing. There are some exceptions. The first one is large companies. For example, if we could convince IBM to use FreeBSD with a DE of their choice to be the standard OS for the laptops distributed to their employees, then IBM would build a large department to support its internal users, and would put a group of internal developers next to it to improve the software. This exists today (and did even before the RedHat acquisition), and IBM calls it the LTC, or Linux Technology Center. It has thousands of employees, with hundreds or thousands of developers (including dozens or more kernel developers). Together with companies such as Intel and Google, IBM is one of the largest contributors to Linux (both in terms of code and in terms of $$$ to the various foundations). The problem is: How to convince a large company to use FreeBSD in that manner?
The second exception is that a very small fraction of users are actually capable of helping with development tasks. One example: I used to work in research and advanced development, partly on a software product that is sold for 6-digit amounts. Occasionally, I would help with difficult support problems. Part of the software product was distributed as scripts, and a part was open source, so customers had access to a small fraction of the source. We had one customer who would occasionally file bug reports that included patches for the source code they could access, or scripts that were canned work-arounds for bugs or misfeatures. But this kind of "user turned into developer" is exceedingly rare. If you look at people who use Linux as a desktop OS, I would guess that 99.9% are not skilled C programmers.