In truth, insects can have multiple heart chambers, there is no set number but being realistic, the average number is 4 or 5 chambers. All of the chambers are near identical as an insect's heart is extremely simple compared to a human heart and is in essence a modified dorsal blood vessel.
The abdominal portion of this blood vessel is the insect's "heart" and is divided into chambers separated by small valve-like openings called "ostia," through which blood enters the heart.
As said in the name 'circulatory system' blood circulates from the heart, flows freely around the body cavity (hemocoel) before entering the heart again through the ostia. The movement of blood back into the heart (through the ostia) is what causes the "pump like" action that keeps the blood flowing (both out of and into the heart)
I hope this answers your question :)