In $ISI$ club each member is on two committees and any two committees have exactly one member in common . There are 5 committees . How many members does $ISI$ club have????
1. We are given that each member of the ISI club is on exactly two committees, and any two committees have exactly one member in common.
2. There are 5 committees in total.
3. We need to determine the number of members in the ISI club.

To solve this, we can use the following reasoning:

4. Consider the set of all pairs of committees. Since there are 5 committees, the number of ways to choose 2 committees out of 5 is given by the binomial coefficient:
   \[
   \binom{5}{2} = \frac{5!}{2!(5-2)!} = \frac{5 \times 4}{2 \times 1} = 10
   \]

5. Each pair of committees has exactly one member in common, and no two pairs of committees can have the same member in common. This implies that each pair of committees corresponds to a unique member of the ISI club.

6. Therefore, the number of members in the ISI club is equal to the number of pairs of committees, which is 10.

The final answer is $\boxed{10}$.