There is never a guarantee of 100% unless you purchase point-of-lay or started pullets.
Point-of-lay or started pullets are female birds almost ready to start laying eggs. They can be purchased from 17 - 20 weeks of age, depending on the hatchery.
We ordered 25 female day-old chicks (pullets).
The hatchery guaranteed 90-95% accuracy.
Guess what - we have 2 males (cockerels)!
So, even if you order all female chicks (pullets) they won't necessarily all be female.
If you order a straight run, you could get more girls that boys, or more boys than girls, but mostly likely it will be 50-50 split.
regards,
keljonma