I'm building some data storage machines and want to use SAS disks. The backplane I've choosen has some iPass connectors (SFF-8087) channeling up to 4 HDDs. There are 6 of these channels. The problem is: how to connect all the 6 channels to one motherboard/controller card? I was unable to find instruction on how to serialize SAS disk connections. The mobo has 2 mini-SAS (SFF-8087 too) connectors and a dedicated LSI SAS2008 controller.
iPass is just a family of connectors, you need to determine what the actual connectors on the back plane are.
Mini-SAS is officially known as SFF-8087 (possibly but unlikely SFF-8086) or SFF-8088 for the external variety. Each connector contains 4 device links.
From there you just need to google around for cable assembly for SFF-8087 to whatever is on the backplane. There are lots of places to purchase the standard assemblies online.
EDIT - Based on updates to the question.
Your issue isn't cabling. Your backplane is exposing all 24 drives as single ports and your controller only has 8 ports.
Your options are:
Purchase a 24 port RAID card.
Purchase a SAS expander which will allow more drives to be connected to the LSI SAS2008 example (despite the description this is not a RAID card, just a SAS expander taking 4x SAS lanes and allowing 24 drives to be attached).
Find a backplane with a SAS expander built in.
You also need to take a good hard look at the capabilities of the LSI SAS2008 to determine if it's actually going to provide the RAID levels and number of volumes your looking to create. For instance it only supports RAID 0, 1, 1E and 10.