tyrport:
On the other hand I have a stainless steel paddle and more than four or five would cause bruising problems.
I never know where to draw the line in my writing, but I've seen scenes much more extreme than anything on this this site.
Well, presumably a stainless steel paddle would have no 'give' in it at all, so the spanker would have to have considerable expertise and self-control. (However, it would work fine for Wonder Woman paddling She-Hulk's naked buttcheeks, I'm figuring.)
By all accounts--yes, I realize that some claimed severe RL spankees on the internet may actually be posting fantasies, yet I'm doubtful that all of them are--there are RL spanking recipients who endure and even enjoy extremely lengthy and intensive ass-thrashings with multiple implements.
In a fictional story, there are numerous factors which could influence how severe a spanking any particular character would be able to undergo without serious physical damage, including his/her past experience as such a punishment recipient and his/her physical toughness and stamina.
Also, a proficient spanker can make an extended chastisement hurt wickedly and still avoid physically damaging his/her victim's posterior to any significant long-term degree.
As a writer (when I'm not 'blocked' anyway), sometimes I'm just feeling like giving my (usually male) protagonist an extremely thorough bare-bottom blistering from my (always female) disciplinarian(s)--even if it ends up being somewhat unrealistic in its severity, after all it's merely a fantasy so only a fictional character in a made-up account is being spanked that intensively.
Another point to consider is that I personally identify most closely with my spanked characters, as many authors seem to--thus when I describe a preteen boy getting a "red-assed shellacking" from his teenage female babysitter, I'm primarily identifying with that very well-chastised young fellow, not his feminine disciplinarian.
Of course, no Library member is required to read any stories which he/she is uncomfortable with and/or offended by...

--C.K.