Freeform JSON not so much I think, but if you combine it with a schema and strict whitespace/formatting rules, you could get quite a few.
I think there are lots of boilerplate sequences like '":{' or '":[' or '", "', etc - though they might already be compressed into a single token if the tokenizer was trained on enough JSON.
There are also situations where the schema would only allow a specific field name as the next token, e.g. if it was the only remaining valid and required field, or if fields have to be output in a specific order.
Good point, I momentarily forgot about the ability to specify a schema. In that case, you'd have a lot of places where there's only one possible output. Even if you have multiple fields and there are no ordering requirements, typical schemas won't take long to get to a unique field name prefix. If you've output `"us` then `ername"` is likely to be the only valid continuation in many cases.