Hi Louise,
Thanks for reaching out! It sounds like the licenses you already own should be just fine for your multisite install, since you’ll only be activating them on a single site. If you need to network activate a plugin across an entire multisite network, individually activate a plugin on 11 or more multisite sub-sites, or perform automatic plugin updates on sub-sites in a multisite network, then you’ll want to make the switch to multisite licensing. You can read more about the differences between individual and multisite licenses on our Knowledgebase here!
I hope that helps to clarify things a bit! Thanks again, and please don’t hesitate to follow up if you should have any other questions!