How many vbo's should i use?
As many as you need, sounds silly but well, its that way
How should i update vbos data, if the data size is not fixed?
Overwrite and render the same VBO with different data and lengths.
How should i create vbos?
How should i render vbos?
see VBO tutorial
How should i deal with data in vbos that i don't want to render anymore?
create a new vbo and copy the data into it or render only parts of that vbo which is in memory.
To render only parts see glVertexBuffer glTexCoordPointer(just calculate the new pointer and new size based on the offset)
Edit 1:
Using a single VBO for everything feels wrong, because you have to manage the allcation of new vertex positions/texture coordinates yourself which gets really messy really fast.
It is better to group small props into VBO's and batch the drawing commands.
Can i add data to a buffer with glBufferSubData (adding 100 elements to a buffer with size x)?
No, its not possible because the description says updates a subset of a buffer object's data store and a subset is a smaller set inside a set.
Edit 2
A good tutorial is also Learning Modern 3D Graphics Programming but its not VBO specific.
