Hello
Looking for a compatible RAM stick for a Librem 13 v3.
DDR4 32GB 2400MHz
DDR4 32GB 2666MHz
DDR4 32GB 3200MHz
Does the MHz matter?
Possible candidates:
https://www.amazon.com/Silicon-Power-3200MHz-PC4-25600-Notebook/dp/B08VDSS5RD
https://www.amazon.com/gp/product/B086GBF8MF
https://www.amazon.com/gp/product/B086GHXQHH/
Apologies - I know this has been asked before but I am not sure if anyone will reply to a 3 year old thread, therefore starting a new thread.
Approximate procedure to answer your own question:
lscpu | grep 'Model name'
Yes.
If you are lucky, it will negotiate to the best common speed. But you definitely don’t want RAM that is dragging the CPU down. And in any case you might not get lucky.
My guess is that the three speeds that you list are all faster than the CPU can support - so you would be relying on getting lucky and having the CPU drag the RAM down.
The other approach that some people take is:
The two approaches can be combined to get a higher degree of confidence, in particular because there are several different types of RAM (e.g. desktop v. laptop and e.g. LP v. standard).
It may also help to clarify how many sticks you currently have installed and what the capacity or capacities in GB is/are.
Some people might also choose a RAM vendor that has a good returns policy, in case it doesn’t work out. 
The cpu is Intel i7 6500u skylake 2.50 GHz.
There is only one ram slot.
DDR4 @ 2133 MHz; any RAM speed faster than that is a waste of money unless you are have a laptop upgrade path with a newer CPU.
Better yet - no need to open the laptop:
sudo dmidecode
will give you all that information under the Memory Device handle
i.e. mine is a Crucial 32GB DDR4 2133
I can even see the serial # and the part #
That is of course if you already have some memory installed (otherwise you would not even be able to boot the device)