How to assing the dedicated memory for the xen host system dom0 permamently and how to preventing dom0 memory ballooning ? also what is the minimum memory need for the xen host dom0 when it as already with 10 guest OX each with 512 MB?
Here is a good link for SLES that imho applies to all XEN-Dom0s.
Additionally to what Giovanni said, you also have to modify the xen-configuration to disable ballooning. The grub-modification will just hide the rest of the available ram from the Dom0.
For your second question: It does not depend on the number of DomUs how much memory is needed in the Dom0. It solely depends on the needs of the Dom0. So if the Dom0 does not do much, you can reduce memory as you like. My personal feeling is that you should not go below 1 GB RAM. For safety (and since I do some things on the Dom0) I use a setting of 2 GB RAM for our Dom0-systems.
You need to specify it in grub.conf and reboot the dom0.