Can I buy one Yoast SEO Care subscription for all of my Multisites?
As of September 1, 2017, we no longer adopt new customers for Yoast SEO Care.
No, you can’t. Yoast SEO Care should be bought for each domain. It is not possible to buy just one Yoast SEO Care for a Multisite WordPress installation and get, for example, five domains taken care of.