I don't want to discuss the difference between languages and dialects. German is a pluricentric language, which means, that there is not "a" single German. Through all the centuries from proto German up to now there never was only one German language or dialect. The situation is similar to the many branches of Arabic language, or Portuguese or also English.
The majority of linguists see Bavarian as one big group of German dialects, but maybe some other have reason to define it as a Germanic language. This situation is opposite for Lower German (spoken in the north of Germany), which is classified as a unique language by most linguists, while some say its a group of German dialects.
Look at former Yugoslavia: In the 1990ies most of the people there spoke one language which was called Serbo-Croatian. Today, we count 4 languages that was classified as variations of Serbo-Croatian 25 years ago, although they didn't change very much in those few years: Serbian, Croatian, Bosnian and Montenegrin. The linguistic situation didn't change much, but the political situation did, and so within a few years one language became four distinct languages.
I don't know if the difference between Bavarian and other German dialect groups is bigger or smaller than the difference between the languages spoken in former Yugoslavia, but you can see, that it often can be complicated to say what is a dialect and what is a distinct language.
How different is Bavarian from Standard German?
Standard German has four grammatical cases, Bavarian has only three, some say it even has only two cases. This of course is oversimplified, but genitive case is used so rarely in Bavarian, that it is not really wrong to say that Bavarian has no genitive case. My first language was a Bavarian dialect, and I had to learn standard German in school, because nobody in the area where I lived spoke standard German in daily life. And so the concept of Genitive case seemed very strange to me.
Des do drüm is n Fodan sei Haus.
- Standard German vocabulary with Bavarian grammar:
Das da drüben ist dem Vater sein Haus.
- Correct Standard German:
Das da drüben ist des Vaters Haus. (outdated word order)
Das da drüben ist das Haus des Vaters. (modern word order)
In Bavarian also dative and accusative case are very often merged together to one case. This is the reason why many native Bavarian speaker still have difficulties to use the correct case when trying to speak standard German.
Des kheat mein Buam.
I siach mein Buam.
- Standard German:
Das gehört meinem Sohn. (literally: ... meinem Buben/Jungen).
Ich sehe meinen Sohn. (... meinen Buben/Jungen).
And, as you can see from the examples above, not only the grammar is different, but also the vocabulary. But still Bavarian and Standard German are very closely related.
Who is speaking Bavarian?
Bavarian is a class of dialects (or maybe even a language?) that is spoken in most parts of Bavaria and in most parts of Austria and in South Tyrol which is part of Italy.
Bavaria is one of the 16 states of Germany. 13 million people are living in Bavaria. But many of the people living in this state are Frankonian who don't speak Bavarian.
Austria is a country that consists of 9 states. In 8 of them people are mainly speaking some bavarian dialect. Austria has 8.9 million citizens, and the one non-Bavarian state is Vorarlberg where 0.4 million people are living. Most of the people living in the 8 other states do speak bavarian dialects, but there are also many people living in Austria who speak other languages or dialects. (0.2 million People from Germany live in Austria, but also many people from Serbia, Turkey and many other countries.)
South Tyrol has 0.5 million people, but since it became part of Italy about 100 years ago, at the end of WW I, there are many people in South Tyrol speaking Italien now.
So, all together the number of 14.4 million people speaking Bavarian seems very plausible to me.
Boarisch (Bavarian) is also one of the languages in which Wikipedia is available. For example there is an article about Bavarian language/Dialects avaiable in:
English Wikipedia contains 6M articles, German Wikipedia 2.4M, while Bavarian has 30K articles.
But Wikipedia is available in 15 different variations/dialects of German, they are listed here: Wikipedia:Sprachen Meist kleinere Wikipedias aus Mitteleuropa (Sorry, the article about language versions of Wikipedia is available in more than 50 languages, but English is not one of them.)