For Flight Simulation:
64G (16×4) DDR4 @ 3600 CL 16
or
32GB (8×4) DDR4 @ 4000 CL 15
Which CPU ? What’s your budget?
32gb x … seems so far to be sweet spot ““and in general,””
the lower the cas (CL #) the better.
I’d search the net for about your mobo see what other have found. Most mobo have their own sweet spots too.
Like most hardware discussions, this can get very opinionated/favorited.
( is favorited a word? 
)
I haven’t managed to get near 32GB, but it does sometimes hit 20. No problems with 32Gb.
11900K CPU with an ASUS ROG Strix Z590-A motherboard.
You can run 3600 at Gear 1 ratio, not sure about 4000 as the timings are not an even number. So the 3600 will probably be faster (if running gear 1)
Also, make sure to enable XMP in bios to take advantage of those clocks. More of a PSA for anyone else looking here.
Both of these are fast enough for MSFS and you won’t use more than 32Gb in MSFS.
This only holds true when comparing RAM clocked at the same speed because the number is a count of clock cycles. For example CL18 timing at 1800Hz is exactly the same latency as CL9 timing at 900Hz