Your question has nothing to do with Qt really... It's more a question from the Software Engineering domain - how to split the system into modules and classes, how to call and reference them. There is no single recipe to doing that. A general rule should be that the naming scheme should be convenient for people implementing the software and clear enough so that if other people start working on it they won't have problems with understanding the structure of the system.

In general a unit name should have a singular form (class should be called Project and not Projects). An instance of that or similar unit can have a plurar form (so you can call a vector of Project instances "projects"). But remember it is just a convention and in practice you can name your classes and variables the way you like and find useful, even if the whole world does it differently.