No city has more Bitcoin nodes than Frankfurt am Main
Frankfurt has the most (registered) Bitcoin nodes in the world.

Frankfurt has the most (registered) Bitcoin nodes in the world. The city also ranks in the top ten for Lightning nodes. You could say Frankfurt is steadily becoming the Bitcoin capital of Europe.
According to data from the analytics platform Bitnodes, there are 652 registered Bitcoin nodes in the financial metropole. In second and third place are Ashburn and Helsinki with 517 and 494 BTC nodes. The number of Lightning nodes is also unusually high in Frankfurt. There are currently 102 Lightning nodes there.
In a country comparison, Germany ranks second after the United States. About 14 percent of all registered Bitcoin nodes are in Germany. That’s roughly half of the 10,000 found in the U.S. A large portion of the nodes operate via the Tor network. For about a quarter of the nodes, the location is unknown.
In the Bitcoin network, the nodes verify transactions and check that network rules are followed by all participants.