Max resolution and refresh rate depends on the connection's bandwidth, the pixel depth, and the compression.
A 40 Gbps Thunderbolt or USB4 dock can provide two HBR2 connections, or one one HBR3 connection with an HBR connection (assuming four lanes of DisplayPort for each connection).
A USB-C dock or hub only has two lanes of DisplayPort if it provide USB 3.x; but if they provide only USB 2.0 then they can have four lanes of DisplayPort.
DisplayPort 1.2 can support 4:2:2 which multiplies max pixel clocks by 1.5.
DisplayPort 1.4 adds support for 4:2:0 which multiplies max pixel clock by 2.
DisplayPort 1.4 supports DSC. DSC can effectively double or triple the max pixel clock. DSC is supported by RTX and Navi GPUs, and Intel 11th gen CPUs with 10th gen graphics or later (Ice Lake, Tiger Lake), and M1 CPUs. When DSC is used 6K 60Hz only requires an HBR2 connection, so you could connect two of those. I think chroma sub sampling (4:2:2, 4:2:0) can be combined with DSC.
Total bandwidths (assuming four lanes of DisplayPort with 4:4:4 or RGB pixels; example display refresh rate is not necessarily the max for the resolution with the connection)
RBR: 5.184 Gbps: 1920x1200 60Hz 10bpc 154MHz
HBR: 8.64 Gbps: 2560x1600 60Hz 10bpc 269MHz
HBR2: 17.28 Gbps: 4K 3840x2160 60Hz 10bpc 533MHz, 4096x2304 60Hz 8bpc 605 MHz
HBR3: 25.92 Gbps: 5K 5120x2880 60Hz 8bpc 938MHz, 8K 7680x4320 30Hz 8bpc 1030MHz
HBR2x2: 34.56 Gbps: Dell or LG 5K 5120x2880 60Hz 10bpc 966MHz
Thunderbolt 40 Gbps: Apple 6K 6016x3384 60Hz 10bpc 1290MHz (this connection uses HBR3x2 but 6K doesn't require all the bandwidth of HBR3x2 and Thunderbolt doesn't transmit the DisplayPort stuffing symbols used to fill the bandwidth so it works).
HBR2 DSC: 51.84 Gbps: Apple 6K 6016x3384 60Hz 12bpc 1290MHz
HBR3x2: 51.84 Gbps Dell 8K 7680x4320 60Hz 8bpc 2118MHz
HBR3 DSC 77.76 Gbps: 8K 7680x4320 60Hz 12bpc 2090MHz
It may be possible for a Thunderbolt cable to transmit more than two DisplayPort signals (e.g. four HBR displays or six RBR displays) using eGPUs that have Thunderbolt controllers with their DisplayPort inputs connected to their GPU, such as the Blackmagic eGPUs and the new Sonnet eGPU Breakaway Puck RX 5500 XT/5700 (I have not seen this tested though - I do know a Blackmagic eGPU connected to an M1 Mac (which don't support eGPUs) will pass the DisplayPort signals from the M1 Mac through the downstream Thunderbolt port of the eGPU). In that case, multiple docks need to be connected to provide the display output connections. An OWC Thunderbolt 4 Hub or Dock can output two displays while having a third downstream port for chaining another Thunderbolt device. Previous Thunderbolt 3 docks can output one display or two displays if it is at the end of the chain. Thunderbolt 2 docks can only output one display even if it is at the end of the chain. There's no reason a USB4 dock couldn't be made to output more than two displays.
A USB-C or Thunderbolt dock or hub can include a DisplayPort MST Hub to allow multiple displays from a single DisplayPort connection. macOS doesn't support MST for multiple displays but can use it for a single display or mirror displays. A MST Hub can convert between a fast/narrow connection and a slow/wide connection. For example, HBR3 two lanes <-> HBR2 four lanes. The example only allows 75% the bandwidth of HBR2 four lanes though. A DisplayPort 1.4 MST Hub that supports DSC can take a DSC signal and decompress it for a display that doesn't support DSC. Using the example with HBR3 two lanes as input, adding DSC compression to that will give full HBR2 four lanes decompressed.
The Apple USB-C Digital AV Multiport Adapter is an example of a USB-C hub that provides a USB 3.0 connection. That means it only has two lanes of DisplayPort to convert to HDMI. The chip used to convert DisplayPort to HDMI in this adapter supports DSC, so it could provide 4K 60Hz 8bpc RGB. Without DSC, it would be limited to 4K 60Hz 10bpc 4:2:2. Cable Matters makes some USB-C hubs that provide only USB 2.0 so they'll have four lanes of DisplayPort input and don't require DSC for a single HDMI or DisplayPort output. I suppose an external MST hub that supports DSC can be connected externally to a USB-C hub (especially if the USB hub doesn't include a MST hub itself). I think the decompression capability of a DisplayPort 1.4 MST hub that supports DSC maybe lower than expected due to pixel rate limits or number of supported slices but I don't totally understand these limits.
Read about the DisplayPort 1.4 MST hubs at https://www.synaptics.com/products/video-interface-ics . Maybe the new CalDigit SOHO dock uses the VMM8210? The HP Thunderbolt Dock G2 might use the VMM5330 but mine doesn't have the firmware for DSC support - the Delock 87737 does have DSC support.
