That is true for the old 3.x devices. Devices running 5.x don't have any limit on the number of channels. If more channels are installed than will fit in the device memory, the most recently used channels are kept on the device and others are downloaded as needed.
I think the OP was asking more about development channels. As stated, you can have only one at a time sideloaded. If you're working on 5 simultaneously, you can sideload the one you currently want to run. You can also only have one developer key installed at any one time, so if you want to package different channels with different keys, you will need to rekey the device before generating a channel package.
http://sdkdocs.roku.com/display/sdkdoc/ ... Publishing--Mark