A pretty well-known fact is that German is the native languages of the so called DACH countries which are Deutschland / Germany (D), Österreich / Austria (A) and die Schweiz / Switzerland (CH).
A little less known is that German is also spoken either as a mother tongue or as a minority language across Europe. This includes Liechtenstein, Luxembourg, the Netherlands, Denmark, Eastern Belgium, Eastern France (namely Elsass, Lothringen) and South Tyrol / Südtirol (northern Italy).
It is also spoken in former parts of Germany that were lost after the second World War and are now parts of Eastern European countries, most notably Poland, the Czech Republic and Slovakia. Other countries include Romania, Hungary, Kazakhstan and Russia. The same is true for Namibia, a former colony, where German is a so called national language.
All of which makes it the most spoken mother tongue in the European Union and one of the in the world. I think, the order has changed since the reasearch was carried out.
The same is true for South America to which many Germans immigrated over time. In Brazil 5 million people speak German, in Argentina 500.000, in Mexico 200.000.
If you want to dive into the number and are really interested in the topic of how German spread over the centuries and why we have such a multitude of dialects, you should read this article on the (good level of German is required). For everybody else there is of course still the version.