I'd suggest starting the Black Clover manga at chapter 232 after finishing the anime. It picks up right where the anime stops and takes the story further. But if you want a better understanding, you could briefly review some key earlier chapters.
It depends on how the anime concluded. Sometimes there are specific indications or announcements about where to pick up the manga. You might also look for summaries or recaps that point you in the right direction.
You can start from the chapter where the anime ended. Usually, there are forums or fan communities that discuss this and can give you the exact chapter number.