Hi,
HikaShop has been tested so far with 40000 products successfully. We optimized it as much as we could, then, it depends on your server.
Of course, if you have several million products on a shared hosting plan, I can guarantee you that it won't work properly: too much data for not enough processing power.
Also, some options are more resource hungry than others so, if you really have a lot of products, some tweaks might be necessary.
The only way to know is to test it with your server to see if it can handle everything properly.If not, let us know and we'll see if we can find a solution.