From your description, it sounds like you are new to running a Moodle class and are wondering if you can use your desktop machine using something like XAMPP to run a single course with, say, a few dozen students. Sure, that should not be a problem, especially if your students are not all trying to load pages at the same time. Even if they are, it might be a little slow at that moment, but it should still work. You should check Moodle documentation on how to tweak it, however, for better performance.
On the other hand, on the type of machine that you describe, if loaded with something like Ubuntu's free Linux server, you can run a whole school with dozens of classes and hundreds of students and perhaps several dozen students loading pages or taking a quiz at the same time. So, in other words, a desktop Windows machine, while it will let you do things on a small scales, is not the best system if you get any larger than I described. You should think of it as a temporary solution to try things out.
The term "concurrent users" means different things to different people. One workable definition is the number of machines trying to load a page that requires basic database access within a 5 second period. By that definition, many courses may peak at only a few concurrent users, and even a desktop PC can handle that for somebody starting out, although few people would recommend that as a long term solution for using Moodle for classes.
Hardware and performance
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