There are no dedicated relay nodes in i2p the way there are in Tor. I2p relies on a bit of social engineering for relay nodes, which is that relaying is turned on by default. So if you are using i2p, you are a relay node by default, and it can reasonably be assumed that most never change defaults. Even if a few do, the rest provide a large relay network sort of for free, but sort of in exchange for the benefit they receive by using the system.