Well Chrono level alchemists are very rare. They don't show up that much, maybe 4 or 5 born every decade, and among them not all survive. 

A.M.R.O makes sure to get to them and begins their specialized training. So those with exceptional skill are put on the track to become a chrono while others are put on the track to become Lost Numbers. 

If A.M.R.O has more than 12 battle ready chronos available, than the 13th is kept on standby, set to replace a Chrono if he or she would perish. Though standby Chrono's are again quite rare.

A.M.R.O is quite organized, the Chrono's age varies from some new while others a few years from retirement while other some where in the middle. They also have "would-be chronos" being taught so as to fit in a rotation of sorts to ensure there won't be too many or too less. 

Also yes if you don't use alchemy, you won't go insane. However I did mention alchemy is an addiction, it is simply not possible to not use it. You might not go insane but the withdrawal will drive you mad.