$ \frac{(2n)!} { (n+1)!}$
(Proof to be Added)
If the nodes are similar (unlabeled), then the no. of distinct binary trees will be the above value divided by the no. of distinct permutations possible for a binary tree structure, which will be $n!$ for a tree with $n$ nodes.
$ \frac{(2n)!} { (n+1)! n!}$
=$\frac{2nCn}{n+1}$
(This is the $n^{th}$ Catalan number)
Counting the no. of distinct binary search trees possible for n nodes, is similar to counting the no. of distinct binary trees possible for n nodes assuming nodes are unlabeled. Hence, this value will also be
=$\frac{2nCn}{n+1}$
($n^{th}$ Catalan number)
Alternatively, for each valid binary search tree, we can get $n!$ binary trees by permuting the vertices, of which only 1 permutation is a$BST$. Hence, the total no. of binary search trees possible with $n$ nodes will be
$\frac{\text{No. of distinct binary trees with $n$ distinct nodes}} {n!}$
= $ \frac{(2n)!} { (n+1)! n!}$
=$\frac{2nCn}{n+1}$
($n^{th}$ Catalan number)
(We can also use the fact that for a given tree structure, there can be only 1 $BST$. Hence, no. of different $BST$s with n nodes will be equal to the no. of different binary tree structures possible for n nodes)
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A very detailed discussion can be found here.
http://brainatjava.blogspot.in/2017/03/total-number-of-possible-binary-search.html