Will the new 21.5" iMac support 16GB of RAM?

The spec page says this, so my SWAG is it'll support 16 GB:

Memory

More memory (RAM) increases performance and enables your computer to perform faster and better. Choose additional 1333MHz DDR3 memory for your iMac.

The more memory your computer has, the more programs you can run simultaneously, and the better performance you get from your computer.

• Select the standard memory configuration to support day-to-day tasks such as email, word processing, and web browsing as well as more complex tasks such as editing photos, creating illustrations, and building presentations.
• Upgrade your memory to enjoy greater performance for more intensive computing tasks, such as video editing and DVD authoring

Your iMac uses one of the fastest memory technologies available today — 1333MHz, Double Data Rate (DDR3), synchronous dynamic random-access memory (SDRAM) — ensuring that the processor is constantly fed with data without wasting clock cycles.

I have the same computer you do, purchased just two weeks ago. It's currently running on 16 GB of SDRAM and doing great. I, too, purchased the memory upgrade from Other World Computing---they've long been a favorite of mine; great products, unbelievable service; competitive prices usually can't be beat---and tomorrow I'm sending the 4 GB of Apple-installed memory to OWC for a rebate (currently worth $11.00, but it's market-based). Check on their site how to do it. It's very simple, very fast, and worth a few moments of your time. You can use the same plastic container and shipping carton OWC used to ship you your 4x4 GB modules.

Have no fear!

That model can actually support 32 GB of ram (if you have $900 to burn 🙂). Apple has a long history of under reporting how much ram some mac models can support. OWC is a great resource to let you know the real ram maximums of macs.