The best way would be to poll about 100 scapers (but the larger the sample size, the better) and ask how much of the item they have and how much they have traded today. For example, I have 11000 nature runes and have not traded any, but my friend sold 3000 today and no longer has any. If you add up all of that and find that, in total, 115k nature runes are held by 100 people, and 11.5k were traded (these are completely made-up numbers), you can make the educated guess that about 209.8m (amount traded) * (115k/11.5k), or 2.098 billion nature runes exist. Note that I completely made these numbers up (except for the amount traded today), and they don't reflect anything.