Divide by 300,000 (300KB) per image: 66000000 requests a month.
That's 2,200,000 per day (assuming 30-day month)
20 TB data transfer is beyond the worst-case, clearly.
It is more close to 5 TB data transfer, 500 KB images, 20 images per catalog allowing some 16,667 daily visitors throughout the month, assuming every visitor scans through one catalog. Pessimistic, granted.
I suppose my real speculation is the number of daily visitors we can expect with some of the popular material.