From Jeremy Jacobs at National Journal:
Republicans picked up 680 seats in state legislatures, according to the National Conference of State Legislatures — an all time high. To put that number in perspective: In the 1994 GOP wave, Republicans picked up 472 seats. The previous record was in the post-Watergate election of 1974, when Democrats picked up 628 seats.
The GOP gained majorities in at least 14 state house chambers. They now have unified control — meaning both chambers — of 26 state legislatures.