Fyi, the Thunderbolt 3 spec mandates that the port have at least one DisplayPort channel available and at least a PCIe x2 interface available.  System manufacturers can optionally wire two DisplayPort channels and a PCIe x4 interface to the port.  I believe the 17 R5 has both of those optional implementations.

As to your question, it depends on whether you use a device that actually supports Thunderbolt, or a regular USB-C device/cable.  If you use an actual Thunderbolt 3 device, such as a Thunderbolt dock or a Thunderbolt 3 to Dual DisplayPort adapter, you'd be able to run dual 4K displays each at 60 Hz because using Thunderbolt means you can tap into both of the DisplayPort channels wired from the GPU.  You could also run a 5K display such as the LG UltraFine by connecting using a true Thunderbolt 3 cable (not a regular USB-C cable) and connecting directly to the system.

If on the other hand you just use regular USB-C, such as by using a USB-C to DisplayPort dongle/cable, you'll only be tapping into a single DisplayPort channel.  With DisplayPort 1.2, that means you can run a single display up to 4K 60 Hz.

In theory the bandwidth in both cases would allow you to sacrifice resolution and/or display quantity in order to increase refresh rate.  For example, a USB-C connection that allows 4K 60 Hz would have enough bandwidth to do 2560x1600 at 120 Hz or 1080p at 240 Hz instead (and with TB3 you could have two of those types of displays), but I don't know if the GPU will actually support those setups.

Lastly, note that the USB-C cable/adapter scenario assumes you are connecting directly to the port rather built into the system rather than trying to run through a USB-C "mini-dock" adapter that has things like USB ports, Ethernet, SD card readers, etc.  Those devices cut your display bandwidth in half to allocate USB-C high speed lane capacity for USB data, whereas a direct-connected video cable dedicates the full USB-C high speed link to video.