She's beautiful! If I Lived Closer..............................anyhoo, in response to your question about normal size litter, IMO I would say 3 to 5 is Normal, but they always surprise us!
Keep an eye on her, and when she has her babies, you will be able to follow her to her nest, and take the kittens to where ever you have the carrier but let her see you take them and she should follow, take them all at once or she will take the remaining kits elsewhere and you will be bottle feeding the first ones you took and you will have to hunt them down all over again.
Actually a big box with out lid with one side removed halfway down so mom could get out but not babies would be great, but only if you are keeping it in a safe place where it won't get wet, or bothered by animals.
Once you have them in the carrier or box and mom is settled in with them she should stay without a problem.
Be sure the kittens are safe from raccoons, skunks, snakes, Dogs, etc.
Good Luck! and be sure to post Pictures!