Mozilla teamed-up with Tor to add high-capacity Tor middle relays to Tor network. Polaris Privacy Initiative was an effort of Mozilla, Tor Project and the Center of Democracy and Technology. 12 relays, all located in the United States, that will help distribute user traffic; the Tor browser is a great way to keep prying eyes from tracking you. The current design is fully redundant. This allows us to complete maintenance or have node failure without impacting 100% of traffic. The worst case scenario is a 50% loss of capacity.
Source: https://thehackernews.com/2015/01/mozilla-tor-middle-relays.html