# When to expand and when to simulate in Monte Carlo Tree Search?

In Monte Carlo Tree Search (MCTS), we start at root node $$R$$. Then we select some leaf node $$L$$. And we expand $$L$$ by one or more child nodes and simulate from the child to the end of the game.

When should we expand and when should we simulate in MCTS? Why not expand 2 or 3 levels, and simulate from there, then back-propagate the values to the top? Should we just expand 1 level? Why not more?