The Interplanetary Transport Network is a network of mathematically-defined paths through our solar system that would require little to no energy for a spacecraft to follow them, albeit at very low speeds. While these pathways would not be practical for manned spacecraft due to the low speeds they require, they have been proposed for use as transport pathways and trajectories for scientific missions.
I know that NASA has indeed used these pathways for a few of their missions, including the Genesis mission and the Solar and Heliospheric Observatory. The Chinese spacecraft Chang'e 2 also used this network on its voyage.
While there appear to be a relatively vast number of these pathways scattered throughout our solar system, not all of them are currently known. How many of these pathways have we documented so far? Are there any official (or unofficial) estimates out there for what proportion of these pathways have been discovered to date?