We are planning to store item hierarchy in the graph and this could result into millions of nodes. Will the query be still performant enough to support this kind of data size? Does anyone has any benchmarks?
If you're asking if Neo4j can handle graphs of millions (or billions) of nodes, then yes.
But as for performance, that depends entirely on your model and the queries you want to run and the indexes you create to support it. If you have a bad model, or poorly constructed queries, or lack necessary indexes to support your queries, then you'll get poor performance. To ensure your model is good and your queries are streamlined, you'll need to share more details about what exactly you're trying to do, what data you're storing, and the kind of queries you plan to run.