What the are the limitations of WooCommerce / Wordpress in a large scale online marketplace? over 50,000 products
High CPU use when using
Automatic Updating. 500 or so products is recommended. And avoid products with many variations.
Otherwise ... update manually each day instead.
50,000 products? Are you making a huge general store and trying to compete with Amazon/eBay? It won't work. And having 50,000 products in a niche store is ridiculous. "More" is not always better. Stick to quality and unique products rather than filling your store with thousands of boring crap products ...
What are the alternatives?
The Original Alidropship plugin. It does not have a problem large product numbers.
Or if you want to forget about Alidropship plugins altogether ... check that comparison/benchmark link above and import products and process orders 100% manually with other shopping cart plugins.
Or if you want to be ripped off on a monthly basis ... go to Shopify/Oberlo.