This vqmod extension will enter the default value in Quantity text area is 1000 when you Add a new product.
Edit its content to change the default number.
I created this extension for opencart 2.3.0.2 and I'm not sure it's suitable for other versions. Goodluck!

Installation:

Simple upload the "default_product_quantity.xml" file to your "public_html/vqmod/xml" folder.