Instructions for how to run more than four motors from a single Control or Expansion Hub using a REV Spark Mini. Westside Robotics have a number of other tutorials available as GitHub repository wikis.
This document from REV identifies the actual gear ratios for ultraplanetary gear cartridges. The naming of each type of cartridge is based on a nominal ratio, and using the exact ratio may be important for things like calculating distance based on motor encoders.
A presentation from "Quantum Stingers", team 13380, that discusses different ways to build mechanisms the expand linearly, both horizontally and vertically.
A presentation from "The Antidote", team 14320, that outlines the different ways that gears, sprockets, chains, belts, and more can be used to transfer motion across a robot.