Hi Village Chapel,
The maximum number of expanders you can have on a single DX port is 2, so you would indeed require either the DX Hub or SLink option card for this.
Depending on your exact needs, the GX4816 may also be interesting to you (note that you can daisy chain extra DX units or a ME system from this, and it would leave your I/O Port free for other option cards).
Something that might be useful is the SLink connections document (https://www.allen-heath.com/media/SLinkConnections_V1_5_0.pdf) which discusses and gives examples of all the possible connections to an SLink port.
Thanks!
Keith.