We are brand new as well, but I don’t think you have to get extra. It’s definitely not guaranteed you will lose some. Maybe err on the side of six, but you also don’t want more than you can handle in case everyone is fine.
I’m not trying to scare you, but things do happen with chicks. You just need to make sure they have the right temperature, food and water, you watch out that their butts don’t get poopy (called “pasty butt” it can block them up and kill them). We fed ours medicated chick food and added a little apple cider vinegar into their water (the water should be nice and warm). This worked for us, as well as probably a little luck.
I’m not sure on different breeds, although j do know you sometimes need to be careful mixing more aggressive types with more docile types so the docile ones don’t get picked on. Buff Orpingtons are supposed to be really friendly and tame but don’t mix well with Rhode Island Reds which are more aggressive
Bantam Silkies are very docile and friendly as well. They are one of the most common chickens for pets (they are going to be pets). Red Stars are supposed to be docile. Ours are but they aren't very friendly not even to each other. We aren't going to mix them or else the Red Stars would kill the Bantams.