Update
Ok Learning, here it is a solution..
var tot_variants_for_test =
(from v_name in
(from t_op in test
select new { v_name = t_op.TestOperation.Select(sv => sv.SubVariants.Variants.Name) }
).First().v_name
select v_name)
.Union(
(from v_name in
(from t_opdf in test
select new { v_name = t_opdf.TestOperationDifference.Select(sv => sv.SubVariants.Variants.Name) }
).First().v_name
select v_name))
.Count();