Hello, in the organization, when you deactivate the user, you can change the owner for all objects of the departing user to a new user, then you decide what to do with the deactivated user to delete or leave the user
Best and suggested approach is to
If you are limited by # of available licenses (i.e. you don't have any free license available), then you won't be able to transfer ownership of departing users' objects to new user without adding new user first. But you can't add new user without deactivating departing user first. In that case, if you must, transfer object ownership of all objects owned by departing user to one of the already existing user.
I prefer not to transfer ownership and do it later on case by case.