[Tristan Lall]

Two-speeds are pretty common (e.g. SuperShifter, many custom ones), and three-speeds are found occasionally (e.g. Tigertrons' ball shifter, DeWalt transmission), but four-speeds are rare (e.g. Killer Bees' transmission). I don't know of any with more ratios than that, except if we count CVTs (e.g. WPI's toroidal CVT).

As you increase the number of discrete ratios, you risk spending too much time shifting (with your wheels are disengaged), and not enough time driving. Also, most shifting mechanisms use either servos, which are relatively weak, and tend to be slow, or pneumatic pistons, which are fast and strong, but very bulky and generally only useful in two positions. As you increase the number of mechanisms that need to be shifted, you risk increasing the complexity to unreasonable levels.

Typical design and usage of a FIRST transmission involves one low ratio for pushing other robots, and another high ratio for driving. While it's possible to have other ratios, there is rarely a need for full power at medium speed, and consequently you can just ease off on the throttle if high gear is too fast for sufficient maneouvrability.