I ordered my i5 Mini from Amazon and it came with 8GB of RAM. Here are my questions:
1) Do I need more than 8GB if I don't do much multitasking?
2) Do I need more than 8GB if Activity Mon says I'm currently at around 4.6GB as I type this message?
3) If I increase the RAM amount will that decrease or almost eliminate disk writes to the SSD via Activity Mon (currently 867MB)?
4) The i5 Mini is pretty responsive, will the system get even faster with more RAM?
I know you already did your upgrade, but nobody seems to have offered any technical advice. I thought a few bits might be of interest to you.
Regarding your questions:
1) Memory and multitasking - it really depends more on what types of things you're doing with your computer, compared with how much multitasking you're doing. I also use 32 GB in my computer and a single photo-editing application can take up 10-30% of my memory on its own.
2) The amount of memory in use isn't really a useful metric anymore. That's because starting around 3-5 years ago (I can't recall exactly when Apple implemented it) the system began to aggressively use RAM to cache files.
This is a good thing because it speeds up your overall system. We used to think that high RAM utilization was a bad sign and meant that you needed an upgrade, but with modern macOS, unused RAM is wasted RAM. Case in point, with my 32 GB setup and only minimal multitasking (Safari, Mail, iTunes, and the host of background monitoring apps I run that don't take much memory), the system is currently using close to 14 GB on cached files.
This doesn't mean I need an upgrade! If an application demands memory then macOS will write the cached files back to the disk or purge them. The more memory you have, the more unused memory you'll have, and macOS will put it to work for you by caching more onto it.
Using the Activity Monitor,
@velocityg4 offered the good advice of looking at your "memory pressure." If you prefer numbers to graphs, you can look at "memory used," which describes how much RAM is being used by your programs. Basically, if your "memory pressure" is in the green, you should be fine on the RAM front. If you hit yellow from time to time you'll likely still be fine, too. If you're hitting the red regularly then you definitely need more RAM. Of note, I have had some slowdowns even when "memory pressure" just showed an elevated green graph, but I'm not 100% convinced that the problem was purely in RAM. What I can say with certainty is that if your "memory pressure" is a very small, green graph, a RAM upgrade is likely to be a waste.
3) Increasing the RAM should theoretically decrease disk writes (page outs) but it won't eliminate them completely, based on what I see from my system. I can't say with certainty why they're not eliminated entirely.
It's also worth noting that writing to the disk isn't the performance bottleneck that it used to be with spinning, magnetic hard drives. Paging out (writing data from RAM to disk to make space for more things in the RAM) was slow and would slow down other disk operations; modern SSDs are so fast that you usually don't feel it when data is shuffled between the SSD and RAM. It's still nicer to have data cached in the RAM, as it's still faster and does prevent some wear on the SSD (although nearly all consumer SSDs will far outlive the computers they're included in). But reducing page outs used to be some miracle performance boost for systems, and SSD-equipped systems won't perceive such huge performance gains because the SSD was already compensating so well.
4) Speed improvement with RAM really depends on what you're doing, and whether you were RAM-constrained to begin with. Based on your observations, it sounds like you weren't RAM-constrained.
