"If I buy the 16GB upgrade, which is 2 sticks of 8GB, which slots do I stick them in? There are XMM1 through 4"

Use slots XMM1 and XMM2 if you need to use two memory modules

"will the system support 24GB?"

No, but feel free to try and see if you can get it to boot

"I am confused that maybe only 16 and 32 GB are supported"

8, 16 and 32 GB modules are supported in the pattern shown in the posted table

"So, net, do I just plug in one of the 8 GB sticks into XMM1 or 2"

Yes,  and a suggestion would be use the two 8 GB from the kit, better odds that they will work well together due to possible manufacturing revisions.

As shown in the Alienware Aurora R10 service manual--

R10 RAM configuration per service manual

Disable XMP via f2 on startup (not awcc), unplug the pc from the wall, remove the CR2032 coin cell battery and discharge flea power by pressing the power button, put the new RAM both in the same color tab (both black or both white), and the Dell RAM in the opposite color, and then boot up. That's the best chance of booting with mismatched RAM. 

But before that try the f12 on boot epsa built in diagnostics, including the long RAM test.  When you're having RAM problems is really a not a good time to add and mix more RAM to the mix, it will only complicate finding the source of the problem especially if your oem RAM is bad.

"We are having some memory crash problems, and as part of that, I thought I would buy some more memory"

This is really the problem that should be resolved first before mixing old and new RAM.  Generally mixing bad ram with good ram isn't going to fix the issues with the bad RAM. Hopefully it's something simple like just an unsupported oc setting.

Now days with the demanding games out require only 16gb of Ram. But 32 gb is recommended if the game has big maps and if he will be playing Multiplayer stuff. Go big with that awesome machine and get 32gb of Ram 2933 MHz and enable Ram2 in the advantance view in AWCC to make run at it avertised speed. He’ll be good for years! 

use slots 1 & 2 , which i think are second from the left and the furthest right slots.

make sure you bought the correct memory. 1.35 memory throws the whole machine out of wack, and wont over clock it.  and will lock out your memory clock at 2933, 2133, or ~3400 depending on which xmp profile is selected in the bios. which wont matter anyway because 90% of your boots will crash first time then retcon back to xmp disabled in the BIOS.

this has been my experience with 2x8GB trident royal 3600mhz ddr4 1.35v 28800 288pin 16-19-19-39

that and even with the OEM hyper fury, the awcc has no control over memory overclocking, everything is done through ryzen master or the bios xmp's or custom timings

my next course of action is finding 1.20v memory geared towards ryzen not dell's trash oem tunes

side note i went with trident royal for superficial purposes, and found out the voltage thing the hard way. of which some would say that should matter none, however i think Alienware has locked out the dell bios so it only runs within manufacturers parameters regardless of what you try to tell it.

Thanks to all for reply, this is my attempt to let all know at once!  It is very nice to see how kind folks are.

I guess I have two issues:  a current memory issue (Windows stop error: memory blue screen) which happens in the middle of Minecraft, etc.  Not enough memory.

So, I will try to work with Dell support a bit more on the first, and on the second, I will go with 32GB and replace out all the memory, seems best route, both future proofing, and likelihood of success (matrix allows 8, 16, 32 and don't mix the memory).

Again, thanks so much to all.  I have an 11 year old that wants this computer gone, he is so cranked up about the crashes.  Hopefully, can get it smoothed out for him.  Covid era cranky, him....

Thanks again, all y'all….Steve

best of luck. unfortunately with windows, that one error crash code could be 1 of 20 different issues that register to that reference code.