Hi Dana-
Thanks for the excellent question. The short answer is that you can use the same License for unlimited sites within the same business. For example, you can use the same key on a development, staging, and production website.
For design professionals like you, what I generally recommend is this. You can design your client sites using the pro license, and when you deliver it to your client you can replace your license key with a free key.
All the work you’ve done with Pro features will remain, but your client wouldn’t be able to use the pro features moving forward, but they could upgrade in the future if they ever need to.
You shouldn’t share your license with clients after you’ve delivered the site, however, because it can be used for things like purchasing stock images via BoldGrid Connect Search.