I've personally never done more than one. I think the rationale for more is to try to get the disk past infant mortality. Like most electronics, if it doesn't fail early, it may last until obsolete.
But if the rest of your array is healthy and you are diligent you should be able to survive a single disk with infant mortality. Do you have Notifications setup to alert you by email or other agent as soon as Unraid detects a problem? Do you have backups of anything important and irreplaceable?
The original purpose of preclear was to allow you to clear (zero) a disk so it can be added to a new data slot without invalidating parity. If you try to add an unclear disk to a new data slot, Unraid will clear it so parity is maintained.
Older versions of Unraid took the array offline while it cleared a disk, so preclear was invented to clear the disk before adding it to a new data slot. Later version of Unraid clear the disk without taking the array offline, so that original purpose isn't really necessary anymore.
And, note that except for adding a disk to a new data slot, Unraid doesn't need a clear disk for any other scenario. Such as replacing a disk, since the disk contents would be completely overwritten by the rebuild anyway.
Additional functionality of preclear includes some things that also test the disk. but it is possible to test a disk using other tools, such as badblocks or one of the utilities provided as free downloads by the disk manufacturer.