I install subversion to share calendar with sunbird. Every time a user svae a change a version is save.
That is the nature of subversion.
My question is; How many version subversion old and how could I delete the one I don't need?
You can not (at least not easily) as that is the nature of a program like subversion. All source code management software is meant to keep track of changes and a core feature is to restore previous versions.
The only way you can delete older versions (permanently) is to make an export and use the subversion administration tools to remove the revision you no longer need and then re-import the dump.
I am curious to know which reasons you have for storing them in a subversion repository when you really do not want the history tracked. If you do not want to store older versions I suggest you do not save your calenders in a subversion repository but in a normal ibay.