I do 60 at a time, 5 rows of 12 RTTs will fit on a square the size of 9 road tiles each side and group select to sell in one.
The biggest trick for preventing crashing is don't do too many batches or farm if you've done a lot of other things too.
The game seems to keep a note of everything you tap / move / earn and if that list gets too big it goes pop. Just head to Friends Towns, quit the game and restart before you farm and if you farm a lot shut it down again after a while.
Doesn't help with the risk of Bart spoiling the party though !!
