You should be able to use "Query JSON with SQL" to query results with SELECT COUNT(ID) FROM {{ AirtableQuery.data }} or maybe SELECT COUNT(ID) FROM {{ formatDataAsArray(AirtableQuery.data) }}.
If that's not working, try searching around the forums for the topic. There's a bunch of into out there, but if you can't find the answer please start another thread with screenshots of your API Query, what the resulting data looks like (show the output tab), and how you setup your "Query JSON with SQL" resource query.