Like putting 4 of the same card in the same slot on a page? Well, I don't personally do it, because I feel it stretches the slot, and could rip it. But I don't see why it would force the cards to bleed through each other.
The problem with putting multiple cards into one pocket is that, as someone else said, it streatches the pocket out... making it loose... meaning that the cards could easily slip out. Personally, I wouldn't reccomend more than two cards per pocket...
I dunno maybe they'll get hot? I Wouldn't even put two cards in the same pocket beacause the pocket gets stretched and therfore the cards fall out and therfore get damaged.(Wow that last sentence was kinda like the Yu-Gi-Oh! TV show!)
well anyway the number of my cards to a pocket varies like if i had like 2 of a card, those cards go into one pocket. if i had 20.....well needless to say my slot broke.....(with exception to the staryu, that was amazing how it all fit)