I'd open them now. ISA browns could start to lay real soon. You want their first egg in the nest box if you can manage it. They are reatures of habit. If they get in a bad habit about where they are laying, it could be a little tough to break.
The other reason to open them just a bit early is if you find you have a problem, you have a little time to correct it. The type of problems I'm thinking about are maybe they want to roost in the nest box. Or maybe when they are playing around, you find the fake eggs and maybe the nesting material on the floor. That would mean you need a taller lip on the front of the nest to keep the nesting material and eggs from getting scratched out.
I suggest you put a golf ball or some other fake egg in the nests to show them where to lay. That does not always work but most of them can take the hint.
For ten hens, I'd suggest three nest boxes. You could probably get by with two, and they will probably all want to lay in the same one or two anyway, but two is the absolute minimum and three gives you a bit of flexibility. I know ISA browns are not supposed to go broody much, but if one does and you only have two nest boxes, you could have a bit of a problem.