foreign key: set of attribute reference to primary key or alternative key of same table or other table.
1000 2000(here A value cant be different from R1(A))
R1(A(primary key) B C) R2(A(foreighn key) D E)
minimum : consider all A in R2 are different but after 1000 different A they have to repeat since R1(A) are unique then each value of R2(A) is matched twice in R1(A).
R2(A) value can be NULL if you don't consider R1(A) as Primary key.
X= number of record = 2000 or 0
maximum : consider all A in R2 are same then matched to only value of R1(A)
Y=number of record in join = 2000
X+Y = 4000.or 2000