Can I, within a Solr function query, count the number of values in a multivalued field? How would I write a function query that returns documents with, say, 3 or more values
Here's the function query reference, and it doesn't list anything like that, so I think it's safe to assume that there's no such thing.
If the value count is somehow relevant in your case, add it as a separate int field, then operate on that field.