Hi David.
Thanks for your interest in Event Tickets Plus (ET+).
You can use a Personal (1 site) license if ET+ will only be active on 1 site in your multisite network. Same goes for Business (3 sites) and Agency (10 sites), etc… the Multisite License is for more than that many sites all on a single multisite network. However, you may still see that nag message unless you actually do have a multisite license key (sorry about that).
Last but not least, we have a pretty great Refund Policy so you can buy one or many of our add-ons and thoroughly test them.
Let me know if you have any follow-up questions.