In Rome, many houses have large numbers. For example, 3187 as on photo. It is obviously not the post address of the house but something else.
What do they mean? Is it a number in some catalogue?
Those are plates that in the 19th century identified individual gaslights, which were installed in Rome from 1854, starting with 44 streetlamps, and then proceeding to install thousands of them. On the nearest building wall the corresponding number on a maiolica plate was affixed, to uniquely identify that particular streetlamp.
Later the gaslights were progressively substituted with electrical ones, possibly in a different location, but the numbers remained.
The story is told in a blog devoted to Rome (in Italian).