You will need to first prime the walls to seal
the fresh mud on the drywall.
I suggest 2 coats especially if using a low LRV (light reflectivity value) which is usually stated on the paint chip or color wheel. The lower the LRV number e.g., from 1-100, the darker the color. As far as how much paint to purchase, that will vary by manufacturer .
If your colors are higher than say 75 LRV you may be able to get away with just 1 coat. You can always buy more paint if it does not cover to your satisfaction. Also, if the room as very good
natural lighting and you have heavy texture, the chances are higher that you will need 2 coats.
Interior Design and Color Consulting