In which Italian city were more than 150 goldsmiths already practicing their trade in the fifth century?

It's Vicenza, a city in the Veneto region in northeastern Italy.

It is enough to know that the first written evidence of the importance of this sector in the province dates back to the early 14th century, with the signing of the statute of "the Vicenza Guild of Goldsmiths", a corporation of more than 150 craftsmen who practised their profession in a population of twenty thousand inhabitants.

https://artsandculture.google.com/story/the-gold-work-of-vicenza-unioncamere/9QVxhtBtZwYA8A?hl=en

I feel compelled to point out that goldsmithing is an ancient occupation dating back several thousand years -- long before both the 5th and the 14th centuries. In Italy, the most significant ancient work was produced by the Etruscans ca. the 8th century BCE. Insofar as medieval goldsmiths' guilds are concerned, the Roman guild was probably founded in the 13th century but is documented as early as the Vicenza guild.

https://en.wikipedia.org/wiki/Goldsmith

https://www.dailyartmagazine.com/gold-jewelry-of-the-etruscans/

https://www.universitadegliorefici.it/en/archivio-storico/

The official document marking the importance of this trade is the Fratellanza degli Orefici (Constitution of the Guild of Goldsmiths of Vicenza), which was signed in 1399 (the late 14th century). At that time, Vicenza had a population of only about twenty thousand citizens, meaning the 150 practicing artisan goldsmiths represented an immense concentration of wealth and industry for a town of its size. Today, Vicenza remains celebrated as Italy's "City of Gold". It acts as a massive global production hub and hosts Vicenzaoro, Europe's largest gold and jewelry trade show.