What mechanical advantage of movable pulley?

It depends on how many times the rope 'goes around' the pulleys. If there are 2 ropes 'pulling up' then the mechanical advantage is double. Think about it this way: the tension throughout the rope is constant, so if the object being lifted is 100 pounds, but there are 2 ropes going up from the movable pulley, then each rope has 50 pounds tension on it, but they are the same rope, so you are pulling with 50 pounds force on the rope to create the 50 pounds tension.

Note that the pulley system reduces the amount of force necessary to lift, but the amount of work necessary is the same. Example: if you want to lift the 100 pound weight by 1 foot, you will use 100 foot-pounds of work. With the pulley system as described, you are exerting 50 pounds of force, and must pull the rope 2 feet to lift the weight 1 foot.