I have this succulent at home and was wondering about the following questions:
- What kind of succulent is this?
- What will the leaf grow into?
- Will there appear more leafs or just a single one?
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.
As @bli writes, probably an Euphorbia (or something from Euphorbiinae), which is a large and diverse group. Maybe its the species he/she cites, but I cannot tell. The leaves will just be regular leaves though, and with enough sun and moisture it will have several of them. The "knotted" structure that the leaves sits on is the stem (which can have very diverse forms/growth patterns in Euphorbia as a whole). In my personal experience (with a small selection of potted species), they often shed their leaves quite easily when the soil dries up. Their flowers are usually very small and inconspicuous (e.g. Euphorbia leuconeura), but can sometimes be surrounded by colourful bracts (e.g. Euphorbia milii).