If you are licensing courses, you should find that the vendor is flexible about meeting your needs. Typically, you will buy a license for X number of enrollments (an enrollment being one learner in one course), and that license will have a time delimiter as well as the maximum enrollment delimiter. So, for example, you would license a course or package of courses for one year for a maximum of 400 enrollments. The effective cost per enrollment will drop depending on volume. Usually, you buy the license in advance, and it is a use-it- or-lose-it agreement.
You can also license on a pay-per-use basis, paying only every time a new enrollment takes place. Expect to pay a fixed cost per enrollment that is substantially higher than on a volume license. You would only opt for pay-per-use if you had very few learners -- course fees are relatively expensive, and you and the vendor incur more administrative load.
There are many in-between license negotiations that you might be able to do. A common one is to buy a small volume license with an agreement that when you exceed its ceiling you can roll into a second discounted license, or into pay-per-use at a discounted rate. Also, if you are interested in a collection of courses, you can buy different volume licenses for different courses depending on your expected usage, and opt for pay-per-use on those courses that are likely to be used by small numbers.
The key is to go into a negotiation knowing what your usage is likely to be, and get your vendor to work with you on building a package that works. Off-the-shelf courses should not have inflexible off-the- shelf license terms :-)