What kind of succulent is this?
Your plant looks like this one: https://es.wikipedia.org/wiki/Euphorbia_heteropoda. It could be this species or a close relative. I found it in the "Monadenium" section of http://succulent-plant.com/families/euphorbiaceae.html, so the exact species likely belongs to this (former ?) genus.

It seems difficult to find it on other versions of Wikipedia, which makes me think that Euphorbia heteropoda might not be its valid scientific name.

The leaf will likely fall, leaving a scar as can be seen on the older parts of the plant.

From the photos illustrating the Wikipedia article, it seems it can have more than one leaf at a same time.