US FAA type ratings never expire, so it's possible to have dozens of type ratings listed on your ATP certificate. In fact, pilots have been known to have their ATP certificates go on for several pages just to hold all the type ratings. They are all valid as long as currency is maintained on them (virtually impossible to do.)
There is no limit on how many can be kept current as far as regulations (US FARs) go. Most airlines, though, place a limit on their pilots maintaining currency on only one or two types at any given time. That's a decision of the air carrier, not the US FAA, though.