What is the Shannon number?

It is a number, named after the mathematician Claude Shannon who started the area of study, which describes the minimum number of different games that are possible in a game of chess. In this context, a game is the exact sequence of plays made from start to finish. The number is about 10^120 (a one followed by 120 zeros), but that includes some that are mathematically possible but involve illegal moves in the actual game. To give you an idea how big this is, the number of atoms in the observable universe is calculated to be about 10^80!

https://www.chessjournal.com/shannon-number/

The Shannon number, named after Claude Shannon, is an estimated lower bound on the game-tree complexity of chess, calculated to be approximately 10^120.