How much ram can I install in my 2017 Mac 20.5?

Note that in the bold text in this Apple article it is implied that not all Apple Authorized Service Providers will undertake this difficult upgrade:

Apple did not intend this model to be opened. The cost of labor to open and gut the computer to access the RAM slots can cost far more than new RAM modules. Some Apple Authorized Service Providers won't do the work becaue clients don't want to pay that much for what should be a simple upgrade.

Why do you want to add RAM? Slowness?

In most 21.5-inch iMacs made between 2012 and 2017, "slow" has nothing to do with RAM and everything to do with the under-spec slow-turning laptop hard drives Apple opted to install. You can throw massive amounts of RAM at a hard drive issue and not make one iota of diffierence in performance.

The iMac hard drive issue usually manifests these symptoms:

• Computer is slow to start.
• Apps are slow to load.
• Once loaded, apps run fine.

If your are getting these, there is an inexpensive external solution that can help a lot. Let us know.

The 4K model can use up to 64 GB in two slots. The standard model only 32 GB in two slots.

The RAM is not user upgradeable so you'll have to contact a local Authorized Apple Service Provider to do the job.  

Zero, 2012-2020 21.5" iMacs do not have user upgradeable RAM.