I think that is a pretty good , perhaps even ambitious, number for a manga. Its almost a 10 year run.... His previous manga had 120 chapters or so.... Is the information accurate though? As the manga is advancing I can't imagine it lasting that long, I kinda figured half that at 200 chapters would make more sense. There is plenty we don't know yet but so far everything indicates meliodas is already strong enough to obliterate a nation on his own, it is a matter of him being willing to do it or loose enough control. It also does not seem like there are multiple enemies in the background nor does this manga seem to be the type to have multiple story arcs with each featuring unrelated enemies.... It kinda seems like there is one enemy working in the shadows....
The info is reliable, he said it in a recent interview.
He could introduce Arthur and his knights, which would be a whole cast of new characters. Then we have the Maiden Clan and the Demon Clan, with the latter perhaps allying themselves with other clans such as the Trolls and the Vampires mentioned so far as enemies previously annihilated by the Sins.
He said 40 volumes would be maximum so it could be less. It seems he has his story planned but I don't know if he said it seriously. 40 volumes is still a lot but I guess it is not that absurd for the genre.
Well plans change but give or take I think he is implying if he had the chance he'd likely be able to write more, but again I don't think this is set in stone probably just a rough estimate with how things are right now (it is popular after all).