Anti-foulers apparently just make the plug act like its a couple steps hotter. I heard the same thing about never using more than 2, but haven't ever heard a convincing reason why not.
BTW for years I ran two anti-foulers on my 8n, which mysteriously had good compression but a lot of oil in the cylinders, with good results - before i started using them, if the engine didn't start in the first few cranks they'd become covered with oil and I'd have to pull the plugs and clean them in gasoline in order to get it to start. However after giving the rings a good MMO soak, and putting in a new set of the "good' plugs (437s?) I was able to remove the antifoulers, and have not used them for a couple years now, and the tractor starts reliably and without fuss all the time now. Though I was going to have to do a ring job but now I think it may last me to the grave without it, as long as the MMO holds out.
Advance stores seem to for some reason have the best selection of anti-foulers at reasonable price.I tend to keep some around since they are useful for other purposes - with a bit of teflon tape, you can force a NPT threaded fitting into the antifouler, tod make all kinds of nifty gadgets.