I have 11 ball lock kegs and have tried many things (notepad file with corresponding numbered kegs, etc) but eventually settled on good old-fashioned duct tape stuck to the side of the plastic top piece of each keg. 1 piece of duct tape says what beer is in the keg and another gets a little line added each time it is filled with a new beer.
When the line count is odd I take all the pieces off, soak them in PBW if needed, inspect the o-rings, and clean all the little pieces more thoroughly. If its even numbered I just make sure it is clean to the eye and sanitize it. When 1 o-ring is bad I replace them all and restart the numbering.
Right when any keg runs out of beer I open and rinse it out with water then store it upside down with the lid off. This makes the more thorough cleaning, before filling, very quick and simple.
You could rotate in an acid cleaner every X number of times if you really want to, although I have found no need to do so yet. This would be different if you were using sanke kegs since you can not clean them with a brush, or you are using some sort of cleaning setup and never even open them.
As far as how long the beer has been in the keg, that is tracked on the brewing software I use. I do not track which keg it was placed into on that software, just that it was kegged on a specific date.
I could see this method working for any number of kegs as long as the kegs never leave the house. If I were distributing the kegs I would probably switch to an excel file (until I got over a couple hundred) and permanently mark each keg with a number, because the extra wear and tear would probably remove the duct tape.