Reducing shown inventory quantity on storefront - Help Needed

Hello, I am looking to see if I can hide a product if it reaches a certain quantity like 20 items or can I set something that makes the quantity show 20 less than what it actually is? Thanks!

For context,

I plan on sourcing all of my products through Doba and it’s basically a supplier directory. The products sometimes show on Doba that there is remaining stock but I have talked to the support and sometimes that number can be skewed. To combat this, and not get angry customers when they order something and no stock is left - I want to set all of my products on my store to show less than what it is - a cushion essentially.

For example, say you see a watch on my storefront and would like to purchase it, the stock amount shows that there are 100 left. In actuality, there are 120 left.

Limitations: I am working with 300,000+ products and I cannot manually update each and every product. I need some type of ruleset.

Theme: Empire

Website: selvence.com

How do you add products into Shopify in the first place? Custom app? Manual addition? Upload?

Doba - a supplier directory app