An underground line has $26$ stops, including the first and the final one, and all the stops are numbered from $1$ to $26$ according to their order. Inside the train, for each pair $(x,y)$ with $1\leq x < y \leq 26$ there is exactly one passenger that goes from the $x$-th stop to the $y$-th one. If every passenger wants to take a seat during his journey, find the minimum number of seats that must be available on the train.

[i]Proposed by [b]FedeX333X[/b][/i]
1. **Understanding the Problem:**
   - The train has 26 stops, numbered from 1 to 26.
   - For each pair \((x, y)\) with \(1 \leq x < y \leq 26\), there is exactly one passenger traveling from stop \(x\) to stop \(y\).
   - We need to find the minimum number of seats required to ensure every passenger has a seat during their journey.

2. **Analyzing the Passenger Flow:**
   - At stop \(i\), passengers who started their journey at any stop \(x < i\) and are ending their journey at any stop \(y \geq i\) will be on the train.
   - We need to calculate the number of passengers on the train at each stop \(i\) and find the maximum number of passengers at any stop.

3. **Calculating the Number of Passengers at Each Stop:**
   - At stop \(i\), the number of passengers who get on the train is \(26 - i\) (since they are traveling to stops \(i+1, i+2, \ldots, 26\)).
   - The number of passengers who get off the train is \(i - 1\) (since they started their journey at stops \(1, 2, \ldots, i-1\)).
   - Therefore, the net number of passengers on the train at stop \(i\) is:
     \[
     \text{Net passengers} = \sum_{k=1}^{i-1} k + \sum_{k=i+1}^{26} k
     \]

4. **Simplifying the Calculation:**
   - The total number of passengers on the train at stop \(i\) can be expressed as:
     \[
     \text{Net passengers} = (26 - i) + (i - 1) = 25
     \]
   - This means that at each stop \(i\), the number of passengers on the train is \(25\).

5. **Finding the Maximum Number of Passengers:**
   - Since the number of passengers on the train is constant at \(25\) for each stop, the maximum number of passengers on the train at any point in time is \(25\).

6. **Conclusion:**
   - The minimum number of seats required to ensure every passenger has a seat during their journey is \(25\).

The final answer is \(\boxed{25}\).