Well, basic math tells us that if there's 10 people who can participate and you purchase 1 slot, you have a 10% chance to win. If there's a 100 slots and you purchase 1, you have a 1% chance to win. Obviously that's not how probability works but hopefully it help you along the way of understanding it.

If you buy 80% of the spots, there's an 80% likelihood that you receive the item. The only way to guarantee it is to purchase all the slots.