No doubt she will be able to give you a good indication when she reaches the half way point or so. Best advice on top of what Ninja said is to explain to your boss and see if he / she is willing to be flexible.
Just be sensible -remember others want to bring their friends & family too, so don't be selfish. When you join your instructor will advise what is considered an acceptable amount. Once you & your classmates give approximate numbers of those confirmed attending, they can then work out how many spare places are available.
My advice: Mum, Dad, Partner & Siblings. Leave the parrot & gerbil at home.