Maximum join of relations
$r1⋈ r2$ is the natural join of $r1$ and $r2$/ That is inner join on the common attribute $R$, where $R$ is the primary key of $r2$. It is Cartesian product of $r1$ and $r2$ followed by selection.

For each element $(a,b)\in r1⋈ r2$, where $a=(p,q,r)$ and $b=(r', s,t)$, we must have $r=r'$. That is, $b=(r,s,t)$. Since $R$ is the primary key for $r2$, for any $a\in r1$, there is at most one $b$ that can "join" $a$. So we will have at most 2000 tuples.

By the way, I could not see why "Suppose all value of R in r1 are same. Then it should be 4499". It is not even an option in the original problem.