How many monitors iMac 21.5 4K can handle?

Well, Im a programmer and I want bunch of monitors.

I have a 21.5 Inch 5K iMac, from 2017

Just bought a DUAL HDMI Adapter from StarTech.com

My intention is to have 3 extra displays because the Mac has 2 thunderbolt 3 ports so in 1 port I connected the StarTech adapter and in the other one a simple Thunderbolt 3 to HDMI adapter

But when they are connected together I have 2 random displays working, I cant get the 3 extra display to work!

In the past I have both thunderbolt ports with the single HDMI adapter and worked fine.

Im afraid that this iMac, due its 4K resolution, can't handle 3 extra monitors.

What do you say? Thanks.

Apple documents the supported displays now from the installed OS:

Check how many displays your Mac supports: Choose Apple () menu > About This Mac > Support, then click Specifications. On the web page that appears, the number of displays your Mac supports appears under Video Support.

I don’t recommend using more than the official, but sometimes people are OK with adapters that split up a display if they don’t care about full resolution or refresh rates as much as maxing out the total square inches of display.

• https://support.apple.com/en-us/HT202351

Another option is using an external GPU. See Use an external graphics processor with your Mac on Apple's website.

From your comments you should expect three 4k displays - up to 2 4K external displays and the iMac integrated one.