Hi 

To achieve your requirements, you can utilize the Discount and Loyalty features in the Sales module of Odoo. Here are the steps to follow:

Go to the Sales module and navigate to the Product menu. Click on "Discounts and Loyalty.

Click on "New" to create a new discount or loyalty program. Select the program type as "Promotions."

In the conditional rules section, click on "Add" to create a new rule.

1. Minimum Quantity: Set it to 20 (or any desired minimum quantity).

2. Minimum Purchase: Set it to 0 if no minimum purchase is required.

3. Products: Leave this field blank.

4. Categories: Select the relevant category for which you want to apply the promotion (e.g., All, Saleable, or Office Furniture).

5. Product Tag: Leave this field blank.

6. Save the rule.

Set the desired reward for the promotion and save the entire promotion.

Now, when you go to the Sales module and create a sales order with 20 or more products from the specified category, you can apply the promotion. In the order, click on "Promotions" and select the promotion you created. The reward associated with the promotion will be applied to the order.

Please note that the exact steps may vary slightly depending on the version of Odoo you are using.

Hope it helps