What does 'Sylvarenum' mean?

The name was chosen rather arbitrarily: it just needed to be a unique, unused string.
Then, "sylva" (or something similar) in Latin means forest, and the suffix "num" somewhat resembles "number", so...

What is the goal of this game series?

It depends on the variant: some are mainly meant for practicing with the Rust programming language, the Vulkan API specification, and other aspects of game development.
The true and final Sylvarenum is the ultimate goal: it will be a proper game, made for those who, in sandbox survival games, dislike stagnation and enjoy exploring landscapes never seen before.

How many variants currently exist?

At the moment, since development has only recently begun, there is only one variant: Sylvarenum-XY: XY because it develops along the X and Y axes (similar to Terraria); it is one of those variants that are more of an experiment than a game.

What will the final game be like?

The final game will be somewhat like Minecraft (or Vintage Story, if you prefer), featuring trees that grow in real time and can be interacted with, different species depending on the characteristics of the environment, plants that spread or die, flora that reacts to seasonal changes, and much more...!