"enough" is a problematic word for GR/EN teaching. While it is often translated as "a lot, much very, much etc" it's better to think of it as as much as is needed. E.g. "We had a lot of food for the party but it wasn't enough." I've seen so many essays/compositons go astray that I began to insist on "as much as is needed" as the first meaning and then of course "a lot etc". (Old school marm you know. :-))) In this sentence, even without context, I'd lean towards a lot just to be safe.
For αυτός γράφει αρκετά the English translation provided is "he writes a lot." It left me wondering whether πολλά could be interchangeable. I checked the comments before posting this, so I hope it's not redundant or obvious. Ευχαριστώ. PS: another post mentions that the two are distinct: https://forum.duolingo.com/comment/26371911