$ S\subset\mathbb N$ is called a square set, iff for each $ x,y\in S$, $ xy\plus{}1$ is square of an integer.
a) Is $ S$ finite?
b) Find maximum number of elements of $ S$.
Let's address the problem step by step.

### Part (a): Is \( S \) finite?

1. **Assume \( S \) is infinite:**
   Suppose \( S \) is an infinite set. This means there are infinitely many elements \( x, y \in S \) such that \( xy + 1 \) is a perfect square.

2. **Consider two elements \( a, b \in S \):**
   Let \( a, b \in S \). Then there exists an integer \( m \) such that \( ab + 1 = m^2 \). Rearranging, we get:
   \[
   ab = m^2 - 1
   \]

3. **Polynomial construction:**
   Consider the polynomial \( P(x) = (ax + 1)(bx + 1) \). Expanding this, we get:
   \[
   P(x) = abx^2 + (a + b)x + 1
   \]
   Since \( ab = m^2 - 1 \), we can rewrite \( P(x) \) as:
   \[
   P(x) = (m^2 - 1)x^2 + (a + b)x + 1
   \]

4. **Square of an integer:**
   For \( x \in S \), \( P(x) \) must be a perfect square. Let \( P(x) = k^2 \) for some integer \( k \). Then:
   \[
   (m^2 - 1)x^2 + (a + b)x + 1 = k^2
   \]

5. **Contradiction:**
   If \( S \) were infinite, we would have infinitely many such polynomials \( P(x) \) that are perfect squares. However, this leads to a contradiction because the polynomial \( P(x) \) cannot be a perfect square for infinitely many \( x \) unless it is a constant polynomial, which it is not.

Therefore, \( S \) cannot be infinite.

### Part (b): Find the maximum number of elements of \( S \).

1. **Smallest elements:**
   Let's consider the smallest elements of \( S \). Suppose \( S = \{a, b, c\} \). Then:
   \[
   ab + 1 = m^2, \quad bc + 1 = n^2, \quad ca + 1 = p^2
   \]

2. **Check small values:**
   Let's check small values to find a set \( S \) with the maximum number of elements. We start with \( a = 1 \):
   \[
   1 \cdot b + 1 = m^2 \implies b = m^2 - 1
   \]
   For \( b = 3 \):
   \[
   1 \cdot 3 + 1 = 4 = 2^2 \implies b = 3
   \]
   For \( c = 8 \):
   \[
   3 \cdot 8 + 1 = 25 = 5^2 \implies c = 8
   \]

3. **Verify the set:**
   Verify if \( \{1, 3, 8\} \) is a square set:
   \[
   1 \cdot 3 + 1 = 4 = 2^2
   \]
   \[
   3 \cdot 8 + 1 = 25 = 5^2
   \]
   \[
   8 \cdot 1 + 1 = 9 = 3^2
   \]
   All conditions are satisfied.

4. **Maximum number of elements:**
   We have found that \( \{1, 3, 8\} \) is a square set with 3 elements. To check if there can be more elements, we would need to find another element \( d \) such that:
   \[
   1 \cdot d + 1, \quad 3 \cdot d + 1, \quad 8 \cdot d + 1
   \]
   are all perfect squares. This is not possible as it leads to contradictions.

Thus, the maximum number of elements in \( S \) is 3.

The final answer is \( \boxed{3} \)