I don't have 12 tennis balls, but I wouldn't even recommend that you had one for each hole. He had lots of fun moving balls from one space to the next and applauding himself after each.
I think that if I had a ball for each space, he would fill them all up and then think the game was 'done' and leave it.
But it was a simple activity we did with materials on hand, and he had a great time. Alternatively, you could use a mini muffin pan, and those tiny little balls from the dollar store if that is what you have around your place.