Minimum Purchase
When considering the number of user licenses to buy, you should keep in mind that you should only purchase enough licenses based on your estimate of how many will actually participate. You always have the option to purchase more licenses but once purchased, there is no opportunity to reclaim the unused ones.
When purchasing a license for your event, the minimum purchase quantity at each payment event is for ten (10) users. After that, you can add individual user licenses in quantities of one (e.g., 11, 12, 13....22, 23...101, etc.).
If after purchasing your first set of licenses for an event, any additional ones you buy later would also begin with a minimum of ten (10).
Since there is always an option to add more licenses, it is usually good practice to estimate participation on the lower side of your expectation. However, in some case you better off purchasing them all up front. This is especially true for smaller events. For example, if you are hosting an event that may have as many as twelve (12) users, it may make sense to purchase them all up front instead since the cost of twelve (12) licenses up front, would be less expensive than purchasing ten (10) to start your event and an additional ten (10) later.