When you first learn about blockchain it is made out to be a single public blockchain that contains all transactions from all users. But when you start considering privacy and performance, it seems you subdivide this into multiple blockchains of some sort.
I'm wondering what some primary examples are of systems (like Ethereum perhaps) that use multiple blockchains, and roughly how many they use. I'd be interested to see if there are any examples that use more than a handful, such as 100, or if any use 1000's, or 1-per user, or millions per user, type thing. That would give a sense of the system.