Following script-background helps you to take a list of CI classes utilized by your CMDB.

findUsedCIClasses();
function findUsedCIClasses()
{
    var count = new GlideAggregate('cmdb_ci');
    count.addAggregate('COUNT', 'sys_class_name');
    count.query();
    while (count.next()) 
    {
        var ciClass = count.sys_class_name;
        var classCount = count.getAggregate('COUNT', 'sys_class_name');
        if(classCount>0)
        {
            gs.log(""+ciClass);
            //gs.log(""+ciClass+" : "+classCount) // CI classes with CI count
        }
    }
}