There are a few 8 track recorders for the iPad, but not a huge number to choose from. Studio Mini XL, is the big brother to Studio Mini on the iPhone/Touch.
Studio Mini XL started off as an app called OctoPod (I believe), and I wasn’t terribly impressed with that as a 4 track, but since then both the iPhone app and now the iPad app have really come on. Apart from a name change which made a lot of sense the interface for both of these apps has been improved and they both now look like recording applications.
Studio Mini is 4 tracks and the XL version on the iPad is 8. Both apps have a loops channel in addition to this. Both a completely serviceable multitrack recorders that you could happily use for any recording project.
The iPad app has a tuner built in, wifi sync and iTunes import, and one of my favourite features, a notes section for your songs which I think is particularly useful.
Both apps have been consistently updated since launch and, in discussion with the developer there are more good things coming to both of these apps, so well worth a look.