number of DLL files

There is no rule of thumb for that.

A reason for using dll's is that a dll can be used in any application without having to share the source code. You can share some functionality and keep it a secret how you have done that. But it still makes it possible to reuse certain functionality/

You can change some functionality without having to recompile and maybe the need to re-install the complete application. Most updates are supplied in this way.

If some functionality changes rather often or it is uncertain if something specific needs to change in the future, you could put that specific functionality in it's own dll. This makes it easy to change, update and also easy to test. You would only need to test the functionality changed in that dll because that's the only thing that was updated. Larger companies love this because they otherwise need to perform a regression test of the complete application.

Like the name says, dll's are also dynamic. You can load dll's at runtime that were unknown at compile time of your application. Of course you would need some ground rules to get started and also know if a dll was created to function with your application. But in this way you it is possible to add plug-in capabilities in your application. This way you give yourself, and could also give other developers the possibility to extend the features of the application.

So, there's is no specific rule of thumb. Simply don't create unnecessary dll's because you would simply make it harder on yourself.

Good luck!