Adding 2 monitors to iMac 27" 5K. How big can I go?

I have 2 identical monitors added to my iMac 27" 5K. I may need to replace them. How big can I go? I mean, what are the rules for what monitors it can support? Thank you!!

Here's my iMac hardware plus one of my two identical monitors:

Hardware Overview:

  Model Name: iMac

  Model Identifier: iMac18,3

  Processor Name: Quad-Core Intel Core i7

  Processor Speed: 4.2 GHz

  Number of Processors: 1

  Total Number of Cores: 4

  L2 Cache (per Core): 256 KB

  L3 Cache: 8 MB

  Hyper-Threading Technology: Enabled

  Memory: 16 GB

  System Firmware Version: 447.40.12.0.0

  OS Loader Version: 540.40.4~45

  SMC Version (system): 2.41f2

  Serial Number (system): D25VJ01MJ1GP

  Hardware UUID: 02AC7BC2-B76F-5BCE-9487-2807FF104A17

  Provisioning UDID: 02AC7BC2-B76F-5BCE-9487-2807FF104A17

Radeon Pro 575:

  Chipset Model: Radeon Pro 575

  Type: GPU

  Bus: PCIe

  PCIe Lane Width: x16

  VRAM (Total): 4 GB

  Vendor: AMD (0x1002)

  Device ID: 0x67df

  Revision ID: 0x00c4

  ROM Revision: 113-D000AA-931

  VBIOS Version: 113-D0001A1P-025

  EFI Driver Version: 01.00.931

  Metal Family: Supported, Metal GPUFamily macOS 2

  Displays:

iMac:

  Display Type: Built-In Retina LCD

  Resolution: Retina 5K (5120 x 2880)

  Framebuffer Depth: 30-Bit Color (ARGB2101010)

  Main Display: Yes

  Mirror: Off

  Online: Yes

  Automatically Adjust Brightness: Yes

  Connection Type: Internal

S2418HN/NX:

  Resolution: 1920 x 1080 (1080p FHD - Full High Definition)

  UI Looks like: 1920 x 1080 @ 60.00Hz

  Framebuffer Depth: 30-Bit Color (ARGB2101010)

  Display Serial Number: KF1HK71D400L

  Mirror: Off

  Online: Yes

  Rotation: Supported

  Adapter Type: DVI or HDMI

  Automatically Adjust Brightness: Yes