Where do these numbers come from?
From the checks people run on this site: whether their address is a datacenter (VPN, proxy) or a home provider, whether WebRTC gave out another address, and whether their clock matches their address. Only daily totals are kept, never an address or an identifier.
What is a WebRTC leak?
With a VPN on, a browser can still hand a site your real address through WebRTC, the feature video calls use. The WebRTC leak test on this site shows whether yours does.