There are two processes:
Make To Order (MTO). If you activate this (routes are archived by default i Odoo), with buy route on the product, you will get a purchase requisition directly connected to your sales order with the full qty. For whatever you have in stock.
It is possible to manual override: Reduce qty on purchase, and deliver partly from stock. But it is manual. But this is the downside of using MTO. It create extra work in cases where the process is not stictly MTO
The other process is usually named Make To Stock. (MTS). Then you purchase into your stock and sell from there. In this process you use reodering rules on the product. You can set minimum stock = reorder point, refill qty. Running the process "Rescheduling" , automatic by default once a day) or manual from the inventory menu. You also have the operation Replenishment to manually follow up what you need.
In the reorder rule you can set replenishment to Auto or Manual. If Manual you must use the operation Replenishment in Inventory. If Auto requisitions will be reated with the rescheduling job automatic.
If you set Min Quantity = 0 you will only get need to replenish when you have sale for more than stock.
Using reordering rules and Replenishment / Rescheduling job is the recommendation for your process.