orders are never closed as customer can buy news days of work that are added to the same order every year.
So is this case manageable in the application? If yes, how?

New days mean new order !.
You deal with a "contract" that have several orders...
We dont' manage contract, but I'm not sure it is important : you can just manage orders that refer in Reference the same contract.

Is it possible to get a counter linked to the order to track the consumption depending on tickets for example?

There is no "stock management" in projeqtior.
But order add validated work to project.
In fact, new order should be copied into a new activity, with validated and assigned days.
Then, each ticket is linked to the activity (through field Planning Acitvity).
You'll have summary of tickets on the activity.

I do not understand your last reply. You say that Projeqtor do not manage Contract, but order can refer the same contract, so is there a contract management feature or not?

No, there is not "Contract Management", no item called Contract Exist.
But you can use the field "Reference" to enter the reference of a contract (or anything else, as you wish)