Impossible to enforce. In my complex 95% Spanish, those apartments that are rented in the Summer months are rented by Spanish to Spanish. I can guarantee not one of them declares this rental income. My (spanish) family members that own several apartments in the complex, aren’t even aware of the requirement to declare and pay tax on the apartment they use for personal holiday use (eg it’s not rented but is a second home).
The notion that people will register apartments for what is maybe 8 to 10 weeks holiday rental a year is laughable.
Apartments will be rented on the black as it were and Spanish life will go on as usual. Some Foreign owners will get caught out and heavily fined. And that will be it.