I believe it's documented in the HPE 3PAR concepts guide, a calculator is available to HPE and VAR's to assist you in sizing this.
Today it depends on the number of nodes involved, the raid type and stripe size as to the number of cages and disks required for cage HA.
e.g.
2 nodes and 7+1 required 8 cages per node pair since you need to stripe vertically and can only lose one disk per stripe in raid 5 (multiple stripes per cage).
2 nodes and 14+2 requires the same 8 cages per node pair since you can lose two disks per stripe in raid 6. Although if there are more available it will use those as well.
Note the below table is per node pair.
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Typically your disks would be added per node to match the stripe size meaning 2 nodes x 7+1 = 16 drives, there are exceptions if you are starting fairly small in which case you can make use of express layout which allows both nodes to share the chunklets across fewer disks.
For parity overhead calculations you just need to divide 100 by the stripe size
e.g.
Raid 5 3+1 = 100/4 = 25%
Raid 6 6+2 = 100/8 = 12.5% x 2 parity = 25%
Raid 5 7+1 = 100/8 = 12.5%
Raid 6 14+2 = 100/16 = 6.25% x 2 parity = 12.5%
Note although you have less parity overhead on the larger stripes it means more cages required and larger upgrade increments in terms of disks in order to maintain cage HA across all capacity.
Port and cage level availability are above and beyond what most other systems can provide but it's not mandatory and you can have a mix of CPG's with different layouts dependent on the data value.
Personally I've never seen a hard failure of a cage but I have seen the case on a none 3PAR system where a firmware update left both I/O modules on an enclosure inoperable. I've also seen a single disk slot on a enclosure fail in another type of array that ultimately required downtime to replace the entire enclosure. Similarly I've seen customers want to remove cages (especially NL) or shuffle cages between racks. AFAIK that isn't possible even on the enterprise systems like HDS and VMAX but as you say XIV is effectively Raid 1 (x) so less of an issue, but a large capacity overhead is required for mirroring which doesn't tend to work for flash. If everything is mirrored to another site and you're DR plan is fairly slick and automated then it's probably less of a worry.