The minimum order is 2,000 credits, which is the $19.99 USD pack tier. Smaller amounts such as 500 or 1,000 credits cannot be ordered on their own.
There is no maximum on the size of an order itself. What can limit you is how many credits you can buy in one payment. See the credit card transaction limit.
If you need a specific amount for an in-game event, message us before ordering and we will confirm what is possible.