Each normal ME cable carries 8 channels, dense ones carry 32. In other words you can attach that many AE "thingies" to the line before it runs out of channels. Each side of the controller block carries 32 channels so say you stick a dense cable on the front and then attach 4 normal me cables to that. Youve created 4 branches of 8 channels. Smart cables hold 8 channels as well but show how many are left with glowing lines, like the dense ones do. Most AE thingies wont connect directly to dense cable- you need to branch off and be careful where cables connect. Using colored cables and cable anchors to physically block connections will help with that. If you need more channels, build another controller block and branch off of its sides.
I think in general, anything that takes up a full block can connect to a dense cable (since it will occupy the block next to the dense cable), while any parts that go within a block space (such as the import/export/storage buses, terminals, etc.) must be connected via a normal cable since the dense cable occupies the entire block space.
Note that most ME devices, while acting as cables, act as
normal cables and thus only hold 8 channels, so you can't run a dense cable line with up to 32 channels
through an ME device - you have to place the device
next to the line.
Also note that an individual ME network cannot contain more than one controller. However, the controller is a multiblock and I believe that any cuboid shape up to 7x7x7 is valid. If you need more than that, build multiple networks and interface them.
Finally, note that you can use ME P2P tunnels to connect parts of your network together without having to run dense cables the entire distance. Instead you just connect two P2P tunnels on the parts you want to connect and then run regular cables between them. Both power and channels (up to 32 channels) will be compacted into one channel (per end I think, so two total) while being transmitted along the line. Note that the regular cable section between the two P2P tunnels counts as a separate network and thus doesn't receive power directly from your main network, so you will need to power it, most likely via quartz fiber from your main network. This also doesn't let you get
more channels, since you still will have the full amount of channels coming out of each end so you will still be capped to 32 channels per ME controller side. The main purpose of using the P2P tunnels is to use less dense cable (and perhaps less energy depending on the length of the run and the number of channels you are transmitting).