Banner stack sizes were capped at 100 prior to Monument to Triumph, there wasn't physically enough inventory slots for you to have more than 5 thousand while they were being sold for Legendary Shards (ended with Final Shape) or prior to Monument to Triumph. When Monument to Triumph launched, they raised the stack cap to 999 and collapsed existing stacks into the new stack size, so having 988 meant you had 9 previous stacks of 100 and a stack of 88.