Thunderbolt is daisy chain only, two ports per device max (except the new Mac Pro with three Thunderbolt busses for six ports). There are no Thunderbolt hubs. There are devices that connect multiple devices to Thunderbolt, but they never have more than two Thunderbolt ports. The iMac with two ports can't use more than two single-port Thunderbolt devices. Any other devices need two ports.
All Thunderbolt devices should have two Thunderbolt port to allow daisy-chaining, but a lot of manufacturers are more concerned with price than usability.
You probably don't have much choice on the audio interface (unless there is a version that uses USB) but there are many two-port Thunderbolt RAID systems. They cost more than the single port systems, but it is often worth it.
If you don't need much graphics performance on the HDMI display, you could use USB.
http://www.newertech.com/products/vidu3dvia.php