Several irrational numbers are written on a blackboard. It is known that for every two numbers $ a$ and $ b$ on the blackboard, at least one of the numbers $ a\over b\plus{}1$ and $ b\over a\plus{}1$ is rational. What maximum number of irrational numbers can be on the blackboard? 

[i]Author: Alexander Golovanov[/i]
To solve this problem, we need to determine the maximum number of irrational numbers that can be written on the blackboard such that for any two numbers \(a\) and \(b\), at least one of the numbers \(\frac{a}{b+1}\) and \(\frac{b}{a+1}\) is rational.

1. **Assume there are four irrational numbers \(a, b, c, d\) on the blackboard.**
   - We need to show that this assumption leads to a contradiction.

2. **Consider the pairs \((a, b)\), \((b, c)\), \((c, d)\), and \((d, a)\).**
   - For each pair, at least one of the expressions \(\frac{a}{b+1}\) or \(\frac{b}{a+1}\), \(\frac{b}{c+1}\) or \(\frac{c}{b+1}\), \(\frac{c}{d+1}\) or \(\frac{d}{c+1}\), and \(\frac{d}{a+1}\) or \(\frac{a}{d+1}\) must be rational.

3. **Analyze the implications of these conditions:**
   - Suppose \(\frac{a}{b+1}\) is rational. Then \(a = k(b+1)\) for some rational number \(k\).
   - Similarly, if \(\frac{b}{c+1}\) is rational, then \(b = m(c+1)\) for some rational number \(m\).
   - If \(\frac{c}{d+1}\) is rational, then \(c = n(d+1)\) for some rational number \(n\).
   - If \(\frac{d}{a+1}\) is rational, then \(d = p(a+1)\) for some rational number \(p\).

4. **Substitute these expressions into each other:**
   - From \(a = k(b+1)\), we get \(a = k(m(c+1)+1)\).
   - From \(b = m(c+1)\), we get \(b = m(n(d+1)+1)\).
   - From \(c = n(d+1)\), we get \(c = n(p(a+1)+1)\).
   - From \(d = p(a+1)\), we get \(d = p(k(b+1)+1)\).

5. **Check for consistency:**
   - Substitute \(d = p(a+1)\) into \(c = n(d+1)\):
     \[
     c = n(p(a+1)+1)
     \]
   - Substitute \(c = n(p(a+1)+1)\) into \(b = m(c+1)\):
     \[
     b = m(n(p(a+1)+1)+1)
     \]
   - Substitute \(b = m(n(p(a+1)+1)+1)\) into \(a = k(b+1)\):
     \[
     a = k(m(n(p(a+1)+1)+1)+1)
     \]

6. **Observe the contradiction:**
   - The above substitutions imply that \(a\) is expressed in terms of itself through a series of rational multipliers, which is not possible if \(a\) is irrational.

7. **Conclude that having four irrational numbers leads to a contradiction:**
   - Therefore, the maximum number of irrational numbers that can be on the blackboard is three.

8. **Verify with an example:**
   - Consider the numbers \(\sqrt{2}\), \(\sqrt{2} + 1\), and \(-\frac{\sqrt{2} + 2}{2}\).
   - For any pair of these numbers, at least one of the expressions \(\frac{a}{b+1}\) or \(\frac{b}{a+1}\) is rational.

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