Hello Scott,
Since these APs are no longer available you will have to go with the newer 802.11ax APs (AP3xx,AP4xx or AP5xx series). This will entail having to make one of those APs the virtual controller to adopt the rest of the APs in your environment. An ANYAP profile will have to be created as we as a provisioning policy. Here are the instructions for your perusal: https://extremeportal.force.com/ExtrArticleDetail?an=000079376
You will also have to upgrade the lot to 7.x firmware and configure the new APs.
Some leg work in required but once done you'd be able to add as many of the new APs as you need.
I hope this helps,
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Christoph S.
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Christoph S.