Hi,
depends on size.

A BK15,FK15 or EK15 are the standard size for 20mm diameter ballscrews. They use a DF pair of 7002 bearings for a stiffness of 100N/um
A FK20 or EK20 are standard size for 32mm diameter ballscrews and use a DF pair of 7204 bearings for 190N/um. The BK20, also used for 32mm diameter ballscrews
but uses a lesser bearing pair of bearings DF 7004 for 140N/um.

I'm not sure what the standard size for 25mm or 28mm ballscrews, but I rather suspect BK20,FK20 and EK20. The EK20 and FK20 use beefier 72xx bearing and are preferred over BK20 types.

The bottom line is that the screw is many times stiffer than the fixed mount and therefore to secure a given stiffness you must choose the correct fixed end mounts largely irrespective of the ballscrew size.

The ballscrews I got and used are 32mm diameter but machined for fixed mounts at both ends and uses two FK20 (DF paired 7204) fixed end mounts. I did not really consider at the time, what that
might mean. If your construction were perfect then both mounts would share the load. My construction, as good as I tried to make it will no doubt have favoured one mount over another, so I don't
think I can reasonably claim 2 x 190=380N/um, but I might guess that I'd get 200N/um.

Craig