Given R1(P, Q, R) and R2(R ,U, V) where R is primary key in R1 and U is primary key in R2.
R in R1 is primary key so it should contain unique values assume 1,2,3,......1500(1500tuples)
There is no constraint on R in R2 so it can contain duplicate values assume 1,1,1,...1(2000 tuples)
Natural join is performed on the Common attribute i.e R
1 in R( R1 table) can map with 2000 1's in R( R2 table).
Hence maximum 2000 tuples are possible in natural join