The total number of sites being spidered is constantly going up. More or less if a site gets more than 50(ish) unique visitors per month, it’s included. But if a site drops below 50(ish) it doesn’t get excluded. So the number of sites being indexed is constantly growing. That number of domains is currently around 30M unique domains (that got at least 50 unique visitors in a month at some point in the last 10 years).