A single CD drive reading at 52X is only 7.8MBps. A 16X DVD drive is 22MBps. So even a maximum read speed on the outer edges of the disc, a DVD drive is only about half the maximum write speed of most hard drives, but it is nearly equal to the throughput for many hard drives when the HDD reaches its inner tracks. So if you were ripping a DVD and the reader had reached the edge of the disc, and the hard drive was almost full or fragmented such that it had to write data to its inner tracks, then having two DVD rips going at once could possibly exceed what the hard drive could handle.
In the majority of cases though I think 2 DVD rips at a time would be perfectly possible with a single hard drive. And frankly, it doesn't really matter how much throughput either device has. It's not like it's a timing-dependent process, if the hard drive gets too busy, the application just stops reading the disc for a moment; nothing gets "lost" like older CD burners when the buffer emptied quickly if the hard drive or CPU got busy.
But you could easily rip 4 CDs at a time with current hard drives, even when writing to the inner tracks on the drive, and most audio CDs don't come near filling a disc so it never reaches full speed so you could probably do even more at once.
Seek times might become an issue with multiple discs, since the hard drive read/write head has to move back and forth for each file's location, it can't just stream all the data continuously.
Of course I'm also assuming that the amount of data being written to the drive is pretty much the same amount that's being read off the CD/DVD, i.e., it's not being decrypted and decompressed into a significantly larger file.
So as far as data transfer rates, I don't know why anybody would claim you need a separate hard drive for each CD being ripped, but there might be something to it when ripping DVDs. Unless you're ripping mounds of DVDs though, it certainly wouldn't be cost-effective to set up a system designed specifically for ripping discs, because eventually you'd run out of discs needing ripping. Using a RAID1 or RAID5 array would naturally help in the throughput area, and might help with seek times. But then it'd be cheaper to just have 5 separate drives for ripping 5 DVDs at a time, assuming bandwidth to the drive controller wasn't an issue.