Integer count = 0;
 map<String, Schema.SObjectType> GlobalMap = Schema.getGlobalDescribe();

    {
    for (Schema.SObjectType Obj : GlobalMap.values()) {
        Schema.DescribeSObjectResult ObjDesc = Obj.getDescribe();
        if(ObjDesc.iscustom()){
           count ++;
           system.debug('Object Name: ' + ObjDesc.getName());
        }
    }
 System.debug('Custom Object '+ count );
    }