Could there be a reason why Rome and later Constantinople were both founded on sites with seven hills? Did seven hills carry any particular significance to the Ancient Romans, perhaps in their mythology or religious beliefs? Or was it just a coincidence?
The Seven Hills of Rome is something of a misnomer, as three of them (Quirinal, Esquiline, and Viminal) are all protruding parts of one landmass. As in the Legendary Seven Kings of Rome, the attraction to seven is probably just that seven is a lucky number.
Once you make the "Seven Hills" famous, it is pretty easy to pick out seven lumps in your city to make the allusion. Note that in Constantinople (Istanbul) three of the seven hills are one large hill (4, 5, 6) and three are outside the walls that Constantine founded the city on! (5, 6, 7).
With this freedom of definition of what a hill is, many cities can and do find seven hills. My hometown, Cincinnati Ohio, did this too.