Large Number of Products

I have a client who wants to house large numbers of products-- in the millions of products. We were thinking about using Zen Cart, but have some questions about scalability. I understand it's possible, the only limiting factor would be the server(s) to store and retrieve the data. Does anyone run a cart with that many products? Any suggestions or thoughts?

I have a client who wants to house large numbers of products-- in the millions of products.
We have 100,000 products with success....
On a shared hosting account, no matter what they may claim(rerad the fine print), there will be issues.
On a dedicated server, tuned for a database intensive application like ZenCart, there should be few issues as it is fully dependant upon the resources available and these can be increased as In storage and RAM.