That depends on the size of your modules. The opening in the kb37 is 543mm/107HP wide. You can put in as many modules as you can fit in. Keep in mind that all the modules together must not draw more than 1.5A on each of the +12V and -12V rails.
The PSU is located under the right edge of the opening. On the rightmost 55mm, you may only install modules that are up to 20mm if the boards on the back go all the way down to the edge of the opening.