How much Ram - Lightroom / Photoshop

What size iMac 21.5 or 27", if it's a 21.5 the RAM is not user upgradeable, however if you have a 27" have a minimum of 32 GB of

RAM. When you buy the RAM please only purchase Crucial.com or Macsales.com RAM, why because imacs are very sensitive to using quality RAM and both vendors supply very high quality products, have good pricing, provide excellent service and have lifetime warranties on RAM.

I have a 27" 2017 iMac with i7 CPU and 16 GB of RAM. The only time I've had issues with PSE 2020 was when I was trying to edit a 300+ MB Photoshop file, using the Smudge or Blur tool. Admittedly the file was rather large, much larger than I ever use normally so I'm not surprised.

Bottom line is you should have at least 16GB of RAM and more if you plan on using large files. If the budget permits 32 GB would be a good number to shoot for as rkaufmann87 recommended. And heed his recommendation of Crucial.com or MacSales.com RAM.

Are you upgrading an existing iMac or getting a new one? If a new one order the minimum RAM and upgrade thru the two suppliers recommended. Also get as large an internal SSD as the budget permits. Don;'t go for Fusion drive.

I’m on the Adobe Photography plan and have a 27” 2017 iMac. I’ve maxed it out with 64GB of RAM purchased from OWC/Macsales. When I have both LR and Photoshop open, occasionally the the RAM usage will go to 40GB and higher. A few operations in LR will occasionally use about that much, such as Merge Photos to HDR.

I've only used PSE since retiring13 years ago so can't give you a 2020 answer. However, understand that RAM is not practically upgradeable after purchase on any 21.5-inch iMac. What you buy will be your "forever RAM."

27-inch iMac allow users to upgrade RAM later. Recommended.

I'm running PSE in a 2017 quad-core i7 iMac 27" on 16 GB RAM with no issues, but I suspect the full version will want more. If you buy a 27-incher with the base 8GB RAM, you can buy two 8GB modules from a trusted RAM vendor and have 24GB and it will only cost half of what Apple charges. I did that with my 2017 and applied the savings to getting a big factory SSD--highly recommended.

Whatever it takes, get the factory SSD. Mech drives are glacial and the Fusion drives have not been trouble-free IMHO, nor all that fast.

You will need at least 3GB of RAM or higher in order to run Photoshop CC and 4GB of RAM or higher for Lightroom CC. The recommended RAM for Photoshop CC is 8GB and for Lightroom CC is 12GB.

All of this information is for computers running MacOS.

Also check MacSales.com for SSD drives for upgrades. They have excellent customer support and very complete upgrade kits with detailed videos on how to perform the upgrade if you plan on doing it yourself.

What model ID iMac and what type of boot drive do you have?

SSD is a solid state drive as opposed to the "old style" rotating platter drives. It's blazingly fast, no mechanical parts to wear out and more reliable.