How close

My first post so dont make too much fun.

How close would you recomomend to plant seeds next to each other.
say i have a container which is 40cm's long, 20cm wide, and 20cm deep.

Also when do they need to moved is there a general rule eg If their xxcm high then their roots are xxcm deep and need to be moved?

cheers

I always start and finish in seperate containers...and a general rule 1 gallon for every foot of plant height.