Multilevel models can be fit even when the number of clusters is relatively low. Small-sample techniques for conducting estimation and inference produce valid results with as few as 10 clusters...
For a great review of this topic, see McNeish & Stapleton (2016, Educational Psychology Review) at https://www.researchgate.net/publication/269336961_The_Effect_of_Small_Sample_Size_on_Two_Level_Model_Estimates_A_Review_and_Illustration
Note: The posted video is an expanded remake of an earlier video on this topic.