Hi Andre:
Maybe I can help a little here.
You need one switch for the door close interlock.
You need two switches for the X and Y homing. Mine are mounted for X on the underside of the makerslide and is actuated by the laser carriage assembly. My Y is mounted on the right hand gantry piece. If you use the DSP you can use "soft limits" which you set in the software for the other ends of these two axes to prevent crashes. You can also install physical limits at these ends to do the same thing if you wish. If you are going to use Mach3 you can also set up "soft limits" in that software.
If you are building the movable Z axis (Which I know you are
) you need an upper and lower limit switch for those.
In my build I use a total of seven limit switches.
Door Interlock-1
X axis-2
Y axis-2
Z axis-2
I think five is probably the minimum if you use "soft limits" for the non-homing ends of your X and Y axis and are adding the Z axis.
Hope this helps. If you have questions let me know.
Regards,
Dick