From their blog:
As you probably know, Session is decentralised. There are currently over 2,000 servers around the world working together to deliver your messages. Each one of these servers is an Oxen Service Node, and together they make up the Oxen Service Node Network. These are specialised servers that stake OXEN cryptocurrency to register on the network; and nodes receive OXEN rewards for performing particular services like routing Session messages.
(They are now switching from OXEN/the Oxen network to their own to be able to better control the features of their hosting network)
800k!!! 🥁