Hi I wanted to know that, can we know y plus in advance (during grid generation)? Thanks

NO. As y+ is function of parameters like the friction velocity (u*) which is dependent on the velocity you generally input in the solver part. The y+ value is not constant and will change with whatever boundary condition (velocity) you input in your solver and also varies with location.

you can calculate the y+ using the parameter u will be inputing in the solver, once u know the y+ value, make mesh such that the first node lies within this y+.

check out FLUENT documentation for y+ calculation