There is no limit for the backups yet beside the limit of max. 10 backups on the backup tab of the website.
ISPConfig stable has no email backup and ispconfig GIT which has a partial implementation of a new email backup function email backup is not usable for any real servers as its just a collection of untested dev code. So you can not have any email backup function if ou installed a production server and if you have a email backup function, then you installed code that is not usable for production yet.
Since i wrote the mailbackup-code for devel and the stable-release, you can try this link. I use the mailbackups for a couple of months in a production environment and didn´t notice any bugs.
