1) It was never "common" and it's not really necessary. You might see a slight difference in some few games that have a particular loading pattern if you put the games on a separate SSD and have the drives in the slots on the motherboard that allow that drive to get the best performance, but unless you're one of those "I gotta get that extra 0.5% FPS increase" gamers then don't worry about it. And other applications won't see any difference either. Just get a drive big enough for all your storage needs and be happy. (In real-world testing, even the difference between a SATA and an NVMe drive is hardly noticeable for normal usage. It only matters when there is a very large sequential stream of data.)
2) How quickly it will fill up depends on YOU - your games, your budget, your other data. How much do you use now, and what games are you planning to install, and how big are they, and how much extra other data might you need to store later? If 2TB might be too small for you now or in the next couple of years, get a 4TB. The only good reason to START with multiple SSDs is price, meaning if a 4TB costs more than two 2TB in the model you want. When you get up to the 8TB range, that does happen. But if you fill up your M.2 slots NOW, you won't be able to just add another drive later to expand your storage, because you'd need to remove an existing drive, so you'd have an unused drive afterward. Of course the board you intend to use has 4 slots for SSDs, so that doesn't really apply to you.
Keep in mind you need to plan for having at least 20% space on the SSD that you NEVER use, AND account for the difference between the capacity that is marketed and the actual binary capacity. You need the extra space so that the SSD's controller can performance maintenance tasks and to ensure you always have space for the pseudo-SLC cache to operate. If the pSLC cache can't work because there isn't free space, you won't be able to reach the speeds that the drive is rated for (and depending on the drive, having even more than 20% free can be better). Manufacturers don't tell consumers about this issue. And the capacity problem is where a "2TB" drive actually only has 1.8TB of binary capacity available to the OS. So if you were leaving 20% free, the most you could actually store is 1.4TB.
3) Don't bother unless you really specifically know that it makes a big difference to how you use your computer. Many of the old reasons for doing so don't apply anymore, or only apply if you are just used to handling your PC's storage in a way where it makes sense. A lot of people recommend doing it so you can put your "personal files" on a separate partition that can be untouched if your OS and application partition gets corrupted, but if you're doing proper backups, that really doesn't make any difference other than saving a bit of restoration time. Beyond that it's often just people that have been using computers for ages and are just used to using partitions and different drive letters for things, while normal and newer users get no benefit and it just makes things more complex.
In addition, I did some testing recently that shows that on some drives, partitioning may affect the way the pseudo-SLC cache functions and is allocated, which could, depending on your usage, result in lower performance in some situations. Probably not a problem that people would run into often or even realize was happening, but still something that exists.