Fifteen squillion fifteen squillions.

To understand this, you need to think a little bit about set theory. Just as you can divide one in half an infinite number of times, meaning 1 is the largest number in the world until you look at it as part of a larger set.

Similarly, fifteen squillion is the highest integer in the world, therefore, you can only have a quintillion when talking about multiple sets.

You can have, say, squllion bugs to fix in each module, which gives you a quintillion bugs in total.