Actually 21 since the bottom three have the same number of speakers. ;) A language isolate is a language that is not known to be related to any other language. How many do you know of the ~20 with most speakers?
Disclaimer: Since it is hard to find reliable data, please don't take the number of speakers figures too seriously. Furthermore, sign languages have not been included because of lack of data. Languages that belong tiny families such as Japonic are also excluded.