How many monitors iMac 21.5 4K can handle?
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.