Hello,

If you are looking for a solution in the standard of Odoo, then:

1. How can we select delivery order(s) and create single invoice to that.

This is not possible. You can create the Customer Invoice from the Sales Orders only. You have option to select multiple Sales Orders and create a single invoice in the Action button

2. How can we invoice specific quantity of a specific delivery order

Invoice creation is same as above. But you can control the invoice creation in Odoo. Go to the Product master and change the Invoicing Policy based on the Delivered Quantity. So, the system will generate the Invoice only for the delivered quantity. Or you can manually change the quantities in the Invoice while it's in Draft status.