In the previous post I cited Fontanelle's biography of Leibniz on his way of taking notes. Fontenelle cannot be entirely correct, however. See, the previous notes on Leibniz on this blog. But see also this Website. Leibniz did many interesting things to (and with) his notes.
There are 150,000 to 200,000 slips. Often, they consist of papers containing notes that Leibniz cut up and then used to write new notes on.