I don't understand what you try to do.
Could you explain with screenshots ?

Hello,

I will take a use case :
- I have 2 projects and 2 templates as below :

- I want to create a new project (project 3) by copying the template1 :
--> As I want to put this new project as a sub-project of another, I click on the list-field "is sub-project of":

--> As you can see above, When I do that, all the projects appear on the list (even the templates).
My question is :
Is it possible to hide in this list the templates (I don't want them to appear) ?

I have 3 other (simple) questions which are not related to the topic above :
1) Is it possible to not increment the WBS value for project's template ?
2) How can I initialize the first WBS value at 1 (my wbs values begin at 232 after I have deleted all projects in the database) ?
3) Is it possible to lock a type of project to a type of profile?

I thank you again for your availability.

Best regards

Is it possible to hide in this list the templates (I don't want them to appear) ?

You can see templates in copy because you manage them (you are admin or allocated to the project).

If you :
- set project type as "Template" to the template projects
- connect as project leader (that is not allocated to the project)
You will :
- see Template in the project list,
- not be able to change it,
- be able to copy it
- won't see it in copy form

1) Is it possible to not increment the WBS value for project's template ?

No, they are projects, like any other project

2) How can I initialize the first WBS value at 1 (my wbs values begin at 232 after I have deleted all projects in the database) ?

Move one project.
It will force renumbering.

3) Is it possible to lock a type of project to a type of profile?

Yes, on profie, restriction of types...