cell raising bar spacing

is there an ideal spacing for cells on the raising bar? also I have seen some bars that are open ended (no bottom bar) any reason for that?

On a national frame you can get 11 base cups across a bar comfortably so 22 with the other bar.
Drone Ranger had one with fewer cells on each row. It does not matter at all.
If you take off the bottom bar it sometimes makes the difference between being able to fit roller cages over the bottom row of cells.
You can work with a single row if you don't need so many cells.
Unsurprisingly, I have one I made from correx which is just a strip of correx with a 2 inch side bar each side.
All held together with gaffer tape.
It works fine.
Mostly I just use old frames with the wax cut out of them.

There is just enough room to have two bars in a standard national frame
It helps if the lower one only has one screw in each end so it can be turned
Attachment 1839
these are push in cups which are quite good

They are not so good if you want to use roller cages though