20 tuples.
In Left Outer Join, beside the tuples satisfying join condition of both tables, the remaining left(First) table with non satisfying join condition tuples from first table would also be selected from the database, however, the attributes for second table would be represented as NULL in these tuples.
In best case: All 10 tuples of r2 inner join with r1 are selected, along with remaining 10 tuples of r1 .
In Worst case : 0 tuples of r2 are selected,along with 20 tuple of r1(with Null for joined r2 attributes) .