How much of the Natsume's Book of Friends manga was adapted into the anime? The anime series currently has 6 seasons. How much of the manga was adapted into these 6 seasons? Are there any differences in the plot between the anime and the manga?
As of 4th season, the anime covers the manga up through chapter 51, which is the final chapter of volume 12 of the Japanese tankouban (graphic novels).

A difference occurs in chapter 36, which is in volume 9.

To date, the manga series has been collected into 19 volumes but it is still ongoing in LaLa magazine. There are also a spin-off 4-panel gag manga web comic titled 『ニャンコ先生が行く！』(Nyanko-sensei ga Iku!) which can be read online for free on the official website, free online novels, 1 published novel, and 3 drama CDs (which were furoku freebies only available with the purchase of specific issues of LaLa).