Trunks and minutes can be used in different ways between your reseller account vs. your customer's account.
Within your account, you select (and purchase) trunks and/or minutes from your provider. These are services you are billed for on your credit card. The services can then be spread out across all of your customer's sub-accounts. For example, you could buy 3 outbound trunks, which would enable you to have 3 simultaneous outbound calls. These outbound trunks can be used by any of your sub-accounts, as long as collectively all the accounts don't use more than 3 at the same time.
On your customer accounts, you can then select how many trunks to assign them. This is a "limit" you are imposing on your users. This can be used to both ensure they don't hog all your available resources, and also to ensure they are paying for what they use. For example, you could have three customers who each get 2 outbound trunks. Collectively, that's 3 customer x 2 outbound trunks = 6 trunks total that you've allocated. But in reality, you're only paying your provider for 3 trunks. This is called "over-subscription" and it's a very common practice in telecom. You're assuming that, despite allowing all your customers to have 2 outbound calls at a time, they won't all use it at the same time. Therefore, you can get away with actually only paying for 3 outbound trunks (to your provider), but you can sell a total of 6 outbound trunks to your customers.
In general, the difference between what you pay for your resources (inbound trunks, outbound trunks, per-minute calls) and what you charge your clients for (that they may not actively use, or that you mark up) is where you make your profit.