I was looking at buddahs teeth last night. I have to admit for the first time ever.

I noticed he is missing some which I think is normal right? He is 8 months and I think those just have fallen out....what really got me concerned is on his top row he has a second row coming in. now before I got him I remember reading sometimes all the baby teeth dont come out and they need to be removed. does this sound like something I should be concerned about? also how do you know if their bite is off?