The maximum number of items you can have in one spot in your bank is 4,294,967,295. Try to withdraw that number of gp, runes, or a stakable item. It will all be withdrawn. However, try to withdraw 4,294,967,296 items and it used to (like two weeks ago) say "you can't carry all that!" this number is because it is based on a binary code, and 2^32 is 4,294,967,296, and since there must be a binary code for 0 of an item as well, you could have 1 less than this. So what happens if you exceed this number? does it make a second stack? who knows.