As you guessed it, 5-10 or even 12 color printing involves using additional inks beyond the original CMYK inkset to produce a larger gamut of colors. However, this will have absolutely no bearing whatsoever on how you produce a document as a graphic designer. These are strictly in place for the printer to hit a larger number of colors that are out of the gamut of a typical CMYK inkset. If you're designing in Adobe RGB then you already have all the possibly colors available to you and the printer is just trying to hit more of those than they could in the past.
We run 6 color machines, which add light cyan and light magenta to the mix. What they do is allow us to hit some of the softer colors like skin tones and pastels that CMYK can print somewhat harsh. Large format Epson machines can run up to 12 colors: CMYK+lightcyan+lightmagenta+orange+green+lightblack+lightlightblack+photoblack+matteblack...this doesn't even get into white inks, silver/metallic inks, neon inks, etc.
However, in the graphic design world, you don't even need to blink an eye about this stuff when setting up files, just keep your artwork in Adobe RGB 1998 (don't let a printer tell you that you should design in CMYK) and let them handle the conversion.