If you read through the posts here, you'll see that the pattern tends to be *either* huge losses, nearing 100%, from shipping and/or some sort of illness *or* a very tiny loss, maybe one or two weak chicks, from shipping and early in the process, with the occasional "I did something stupid" loss later in the game.
So getting a whole ton extra probably isn't going to help much. Getting a couple extra will guard against weak chicks getting lost in shipping. Nothing will help you if you don't take care of them, or if there is a major problem in shipping or serious illness in the chicks. So a small number of extras, no more than say, 5% of your total order, seems to be the statistical winner, anyway.