For instance would the cache store 56 cards if i spent 8500 Hp on heroic? Or can the cache also become full?
You should go ahead and try to find out 
Keep in mind that items in your cache expire in about 7 days
I had about 40 odd in there at one point, they all had about 6 days listed against them until they would expire. Didn’t see any mention of a max limit in the cache.
I am waiting for the store special that gives hp bonus lol. All I use is storm and Thor because my ragnarok, widow and x wolverine need more cards lol.
There’s no practical limit to the cache. Technically, there is. But unless you’re a Saudi oil baron or similar, you’re not going to be able to purchase enough covers to ever reach it.