This seems obvious, until you’re trying to sell large enterprise deals and contract negotiation goes to procurement teams. Who have little idea what your custom units of measurement are, or how to forecast reasonable future capacity planning around team. Meanwhile headcount (seats) is easy for them to understand. With a well established model and hiring planning for the coming 12mo.
Don’t assume the consumer of your product is also the buyer.
Head count makes sense in some spaces/products. I shouldn't have been so definitive. However, the units of measure used should always be something the target enterprise understands. For example, the company I work for in healthcare prices contracts by number of beds, which is a number ever hospital understands. Im so glad I don't have to tell someone that they can't be added to use the product cause their seats are maxed out.
Don’t assume the consumer of your product is also the buyer.