Hi Evangelos,
For what I understood, it's like this:
1) Up to a maximum of 2 properties can be rented by a single owner. Otherwise it must be registered as a business.
So as you have 1 apartment, I understand that you are not concerned here.
2) Up to 90 days rental per year in places of more that 10.000 inhabitants (up to 60 days rental in places less than 10'000 inhabitants) are allowed without registering as a business.
So as you are in Athens as I understand, it means that you can rent your flat up to 90 days a year without registering a business.
3) If renting leads to more than 12.000 euros in the year, one has to register as a business. Otherwise it's not necessary.
So if however your earnings from your rentals are above 12.000 euros / year, you need to register as a business. But if you stay below 12.000, you don't need to.
That's what I understood, but I'm not taking any responsability!
All the best,
Louis