A better comparison is Top Gear, the BBC show that was essentially the same as The Grand Tour. At it's absolute peak Top Gear drew live audiences of ~8 million people. When the BBC cancelled it (after Clarkson hit a producer) it was getting audiences of ~5 million. I'm sure you could double that number including on-demand and foreign audiences. It was a popular show.

To me these numbers show that Amazon have managed to compete reasonably favourably with the BBC, which is famous for being really good at making TV shows, within 5 years of starting to make their own content, they've built a platform that people are willing to use to watch that content, and they've developed a model that audiences are willing to pay for. That's huge.