How do I keep track of my wordcounts for billing?

Use the Reporting feature. It shows how many words, keys, or characters each contributor translated or reviewed, and you can filter by user, by language, and by any period up to 90 days, so pulling last month’s numbers by project works well for invoicing.

Language statistics covers a related but different need. Instead of contributor activity over a period, it shows your project’s current translation progress per language, in the same three units (keys, words, or characters).

Both features are scoped per project and available on the Business plan.