it is advertised as a 'limited edition' box set....
do we know how 'limited'?
do I buy an additional one for prosperity?

RickB
It's a marketing term. Also, each and every item produced is kind of 'limited' - even if it's limited to a run of 50.000 copies.

Both the
Pet Sounds and the
Good Vibrations box set are still available for a very reasonable (sometimes even downright cheap) price. I'm not sure if those ever had the addendum "limited edition" but after 15-20 years both are still easily avaible. The
SMiLE Sessions, too - you still get them everywhere.
Let's just say I' be very surprised if you could not still get an adequately priced MiC a couple of years from now.
