The Shannon number, named after Claude Shannon, is an estimated lower bound on the game-tree complexity of chess of 10^{120}, based on about 10^{3} initial moves for White and Black and a typical game lasting about 40 pairs of moves.

Did you also know there are over 9 million different possible positions after three moves each. There are over 288 billion different possible positions after four moves each. The number of distinct 40-move games is far greater than the number of electrons in the observable universe.