When we have a baby for the first time ( like me), we have a lot of doubts, and the diapers are one of them. To help you, I made an Infographic where you can see which is the size of the diaper and how many diapers a baby uses each day. This way you can make your monthly budget and organize your storage.
Mateus born over the night, so I changed the first diaper at 7 am. I had changed other diapers of my cousins, and at reception center where I did voluntary work, but this was the first diaper change with a newborn baby, and was MY Baby. I was so nervous, with afraid to hurt him, so I took some time, so he decided "baptize" me with pee. 😅
You will know when is time to change the diaper, is something natural. When they are newborn, you should change the diaper before the breastfeeding/bottle and every time you see that the baby have poop or pee. As they grow, you will easily notice that it's time to change, with an average of 3-4 hours, you should change the diaper. But as always say, babies aren't equal, so this can be different from babies to babies.
And you, your baby already "baptize" you with pee?🙈