In the following context SATA only supports a single driver per channel?
What is a channel?
A channel in this context is a single electrical connection. SCSI of old connected multiple devices to a channel and addressed each device through a feature in the signaling protocol on the wires. SATA lacks this as it is purposely a simpler protocol.
Which begs the question, So how to port-multipliers work, then?
This is an optional extension to the SATA protocol that not all controllers implement (it adds cost). When used with a port-multiplier you can actually connect multiple drives to a single channel, as the port-multiplier handles the drive-addressibility.