iMac support for dual monitors?

Please review https://support.apple.com/en-us/HT202351

Your question must be more specific for us to tell you what adapters you will need. At a bare minimum we need to know what year iMac you have and what connections the external display you will be using has. Without those basics the article link above is about the best we can do.

Okay, it's realllly simple to do and if you need more than 1 external monitor that is possible too. Just selecting the right adapters and following the instructions in the link I provided and you're set.