It is hard to make multi-heads Ariocarpus from own-rooted seedlings.
You may need years of ages but it also is no promise.
When you grafted Ariocarpus, it might be a multi-heads plant.
The photo is a own-rooted multi-heads kotschoubeyanus.
I cut lower parts of a very young seedling and grafted on a Hylocereus stock.
When you cut lower part of the seedling and grafted, the scion tends to be a multi-heads plant.
After the scion was a multi-heads, I degrafted with vascular bandles of the stock. The plant relies on own roots currently.