Q: Recommended RAM

Your Mac will not run Lion.  System requirements are sufficient to run the operating system it shipped with, and it is able to run Sierra without any RAM upgrade.  Some software may need to be checked for Sierra compatibility before upgrading, and you should backup your Mac before any upgrade.

Since OSX Lion (the forum you posted in), cannot be run on the Mac you describe, I've requested this post be moved to a more suitable forum such as the general iMac forum or even the OSx Sierra forum.

With that said, its a general rule of thumb to get as much RAM as you can afford or it can take if money is no object.  Its better to have it and not need it, than need it and not have it.

A 2015 iMac can take up to 32GB of RAM.   While the 8GB you mention should be enough for most common tasks, things like video editing, or raw photo manipulation will undoubtedly benefit from more RAM.

Its really up to you how much RAM you add to it.

Unless you plan on some very heavy video or image editing the 8 GB should be good.  I have 8 GB on an older 2011 iMac 27" and run Photoshop C3, Filemaker Pro, Media Pro SE and all of the Apple apps without a problem. 

If you already have the iMac run it and see if you have any slowness issues with any of your apps.  Launch Activity Monitor and check the memory pressure throughout the day.  It it appears in the green and steady like the following screenshot you should be fine:

Be sure to keep a minimum of 10-15 GB free space for optimal system and application performance.  Also never use any "cleaning", "optimizing" or "speedup" apps.  They are not needed and do more harm.  Also don't bother with any anti virus software as they also are not needed and will compromise performance.

If you decide to upgrade your memory I recommend you purchase it from Crucial.com or otherworldcomputing.com.  Those are the two most reliable vendors of memory modules for Macs.   I've found that Crucial is usually a bit less expensive