How to knit armholes

I am knitting my 12year daughter pull over and I want to know how many stitches should I bind off for the arm hole for the backside and open front side?? Plz reply

Welcome to KH!

This is very dependent on the size you are making, the gauge, the stitch pattern, and the type of sleeve you would like. Are you making up this pattern or following a printed one?

Here are a couple of sites to guide you: