How do you find mechanical advantage of a pulley?

A single pulley changes the direction of the effort, but it has
no mechanical advantage.  The output force is the same as
the input force, so we'd say that the mechanical advantage is 1.

If there are two pulleys, with the rope going up and down and
around between them several times on its way from the effort
to the load, an arrangement that I think is called 'block and tackle',
then the mechanical advantage turns out to be the number of
strands of rope that are supporting the load.