Extending on this, I'd say that my reasoning is that you should split files by logical group, and ensure that they stay on topic. It's quite easy when you look at the difference between a CSS or JS file. However, each of those can be split by topic / purpose. You'll then find that there's opportunity for reuse rather than duplication :-)
I do like that you mention that the notion of "many files" is mostly a human construct, and I agree fully :-D
AgentDenton is right in the sense that files in a program should be logically grouped.
To extend on Dian's awesome answer, the amount of files (or the splitting of files in a program) depends on what programming language you use and what the purpose of your program is. Generally speaking, in a production-ready application (or a service, etc), the program should be split into multiple files that are grouped together based on the behaviour they provide.
For example, if you are building an application that reads files into memory and then puts them in a database, it makes sense to split the multiple steps into files that are grouped accordingly (e.g you can have the file load step, the decoding step and the persistence step).
It is quite important to get a feel for splitting up your program into multiple files so that (and quoting Dian here) it can "help you orient by dividing and delimiting". This also applies to functions, packages (or modules) etc. In object-oriented programming (think Java, C++) one good rule is the "single responsibility principle" which states that "every function, class (in our case file) or module should have responsibility over a single part of the functionality provided by the software". This is basically saying that in most cases (certainly not all or not in all programming languages) your files should do one thing only and do it well. Try using this rule the next time you code a small application and see if it helps your code become more readable and clean.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.