If you don't care about saving recordings, there is no limit.
(using mfsbackup with -s option)
Here is an excerpt from my faq page:
To save recordings and expand Single drive setup:
Series 1 Standalone (2x)
Series 1 Standalone UK (2x)
Series 1 DTV (1x)
Series 2 & Series 3 standalone & Dtv (1x)
Using mfslive beta w/ restore -f option
more than 10x depending on how big your drive is and what r value you use.