I have not personally played bank shot but the later gottlieb WOW games usually have a backglass feature which will light up to 5 stored extra balls. Looking at that glass, i am guessing it is the 5 yellow circles in th hanging lamp. As long as there are stand by extra balls, it wont take away one of the normal balls.

The Bank Shot shows the extra balls earned on the backglass as yellow WOWs that light up each time you earn one. The "balls to play" light indicates how many you still have....after the ball in play drains, a WOW light will go out & the "balls to play" should stay on the same #. Am having one related issue with mine, if score is achieved to earn a WOW during the bonus points being awarded (upon ball drain), I get the "knock" to earn a WOW, but before an additional ball is awarded, the game ends. Need to fix that.

Each time you get an extra ball for "WOW", a round yellow globe on the pool table light fixture will light. You can get up to five on any ball in play, and as njpinwiz mentioned, until the last "WOW" is used up, the ball in play will remain on the same number.