The countmoves filter comprises the keyword countmoves followed by either a ―― or a legal filter:
    countmoves ――(check) >=3
    countmoves legal ―― ==1

The value of the countmoves filter is the number of moves in the associated argument filter. For example,

    countmoves legal ―― ==1
means that there is exactly one legal move in the position.

To get the number of children of a position, which is the number of moves, use:

    countmoves ――