"There are very many streets in Rome."
Translation:Plurimae plateae sunt Romae.
I would say it is correct. Duo's answer, "Plurimae plateae sunt romae" sounds like "Very many streets are in Rome" (but not all of them; in Naples we also have a few).
"Plateae Romae sunt plurimae", should translate as "The streets in Rome are very many", which more a less the same as "There are very many streets in Rome".