When players a and b hit off between two tee markers how many club lengths can they go back from the markers?

Two.

From the Rules of Golf, Section II, Definitions.

The "teeing ground" is the starting place for the hole to be played. It is a rectangular area two club-lengths in depth, the front and the sides of which are defined by the outside limits of two tee-markers. A ball is outside the teeing ground when all of it lies outside the teeing ground.