When you start playing the game, you are limited to 3 slots. This is true for the majority of the game.
However, there are a few ways to get more pin slots.
The first way is to find pin slots in the bonus stages. There are usually a few pin slots in these stages, which can be very helpful.
The second way to get more pin slots is to find them in the regular stages. This can be difficult, but it’s definitely possible.
Try finding secret areas or high-traffic areas.
The last way to get more pin slots is to purchase them from the in-game store. This is the easiest way, but it’s also the most expensive option.