Re: Max amount of mounted partitions
The limit for GPT isn't "128-ish"; it's precisely 128 by default, but that limit can be raised to arbitrary values if you decide to do so and use appropriate software to implement it.
As to RAID, it depends on how it's implemented. In Linux software RAID, you can have up to however many partitions the partitioning scheme on the underlying disks supports. You either combine partitions from each disk together and use those directly; or you combine partitions together and then treat them as disks and further subdivide them using the same or a different partitioning scheme. When using hardware RAID, the physical disks are combined together in the disk controller and appear to be a single disk to Linux, so you've got the same limits on the combination disk that you'd have on a conventional physical disk of the same size. I'm less familiar with motherboard-based software RAID (aka "fake RAID"), but I think that it gets combined together in a way that's conceptually similar to hardware RAID, although the device names are different.
Most of this is of basically just theoretical/academic interest. In practice, few computers need more than about a dozen partitions. Personally, I find that when dealing with complex or frequently-changing configurations, Linux's Logical Volume Manager (LVM) greatly simplifies configuration, at least when Linux is the primary OS. (Linux's LVM is Linux-only, so it doesn't help a lot when multi-booting with Windows, Mac OS, or any other OS.) If you need more than half a dozen or so filesystems, and if you frequently reconfigure them to modify your installed OSes, then adding, deleting, resizing, and moving physical partitions becomes a major hassle. The logical volumes of LVM are more easily added, deleted, and resized than are partitions, and there's no need to move them (except across physical devices when adding or removing hard disks). Thus, despite the fact there's an LVM learning curve, actually using LVM is much easier than using conventional partitions in such environments.
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