I really dont think you need more than 2. With
Treasure hunt... you will reveal cards until you draw a
seismic assault and then play it.... then discard your hand and win.
if you have too many
seismic assault's in the deck it would reduce the amount of cards you can put in your hand with treasure find and possibly cost your victory.
More land = more power:)
plus you will muligan until you at least have a treasure find. Even Mulligan down to one to get a treasure find does not prevent the deck from winning.
Another thing to note: as long as
Seismic assault is not counter spelled and is in play. Someone can attempt to destroy it( on the stack use its ablitiy as many times as you need to) and win:)