Projects mean the number of free websites you can build for your clients at the same time. Once the site is ready, make sure to upgrade it to Pro and free space for another project. For example, if you are on the Growth plan, you can have 10 projects max in your account meaning 10 free websites max. Once you've reached the limit, you'll be asked to purchase a Pro subscription for at least 1 project. The number of Pro websites in the account is unlimited, thus unlimited clients.