SCSI Ultra 640 was first introduced in 2003 and has a maximum sustained throughput of 640MB/s (hence the name) which is over twice that of SATA2's 300MB/s maximum burst rate. It uses either standard 68 pin high density connectors, or the slightly newer 80 pin connectors, and supports up to 16 devices on the chain, with each needing its own ID (or LUN - Logical Unit Number), and the last device in the chain needs to be terminated so the signal is reflected back down the cable correctly.
So basically it works in the same way as all SCSI versions and the cables are the same as have been used since Ultra Wide SCSI and then Ulta2 Wide SCSI were first introduced back in the mid 90's.
As for your question about the speed of the interface depending on the number of devices. SCSI is shared bandwidth, much like IDE, so the more devices on the chain, the less bandwidth each device will have. In contrast SATA is device independent so all devices get the full bandwidth (in the case of SATA2 all devices would have full access to 300MB/s each). Therefore you can instantly see the initial advantage of SATA over SCSI once you start to have more physical devices attached to a system.
But you do also need to consider other things and this shows reasons why SCSI is often still chosen over SATA. SATA drives still mostly tend to spin at 7200RPM, whereas SCSI drivers are mostly 10K or 15K, so their seek times are much faster. The other is the physical actual data rate of the drive. Most SATA drives still only have a data rate around 50-60MB/s maximum, so even though they are on a 300MB/s SATA connection they wont be using it all. In contrast SCSI drivers are commonly around the 89MB/s range, with some recent ones supporting 150MB/s! Therefore although SCSI shares bandwidth, if you had 4 drives connected to one interface you would still be able to maintain the full 89MB/s x4 bandwidth (356MB/s total) and even the full bandwidth of 4 150MB/s drives, and when you compare this to 4 devices connected via SATA, each on their own independent 300MB/s bandwith, the SCSI will still be faster as each SATA device will probably only be managing around 60MB/s max, for a total of 240MB/s across all 4 drives combined. So you can see the advantages of SCSI over SATA still.
Obviously if you were to start having more tha 4 devices on a SCSI chain it would start to eat into the total shared bandwidth, especially if you were using HDs with 150MB/s data rate. And then SATA might start to be a better solution. Faster SATA drives are starting to appear with faster spin rates and faster data rates closer to those of SCSI drives so SCSI is starting to look less appealing.
Also with SATA having independent bandwidth per channel, you can utilise something called a port splitter. This allows you to connect 4 physical SATA devices to a single SATA channel on the controller and utilise the 300MB/s bandwidth per channel, shared between 4 devices which is more than enough bandwidth to access 4 average SATA HDs. You could therefore add 4 port splitters to a 4 channel SATA card and have 16 physical SATA HDs connected, all accessible at full bandwidth. You couldn't do this with SCSI.
One thing worth noting is the Ultra 640 is probably going to be the last of the original Parallel SCSI type. Serial SCSI and iSCSI are now taking over, as well as most manufacturers now including SATA instead of SCSI on their motherboards. Serial SCSI has the big advantage of supporting hot swapping and faster data rates. Whereas iSCSI is SCSI over TCP/IP which is idea for use with Fibre Optic cabling.