The number of factors that influence the `IE` are
a. Size of the atom , b. Charge on the nucleus
c. Shielding effect , d. The atomic mass

Explanation

To determine the number of factors that influence ionization energy (IE), we will analyze each of the given options:

1. **Size of the Atom**:
- The size of the atom has an inverse relationship with ionization energy. As the size of the atom increases, the distance between the nucleus and the outermost electron also increases. This means that the attraction between the nucleus and the outermost electron decreases, leading to a lower ionization energy. Therefore, the size of the atom is indeed a factor that influences ionization energy.

2. **Charge on the Nucleus**:
- The charge on the nucleus, also known as nuclear charge, directly affects ionization energy. A higher nuclear charge means a stronger attraction between the nucleus and the outermost electrons, which results in a higher ionization energy. Thus, the charge on the nucleus is a significant factor influencing ionization energy.

3. **Shielding Effect**:
- The shielding effect refers to the phenomenon where inner shell electrons repel outer shell electrons, reducing the effective nuclear charge felt by the outermost electrons. A strong shielding effect decreases the attraction between the nucleus and the outermost electrons, leading to a lower ionization energy. Therefore, the shielding effect is also a factor that influences ionization energy.

4. **Atomic Mass**:
- Atomic mass does not directly influence ionization energy. While it may correlate with the size of the atom, it does not have a direct effect on the energy required to remove an electron from the outermost shell. Thus, atomic mass is not considered a factor that influences ionization energy.

**Conclusion**:
The factors that influence ionization energy are:
- Size of the atom
- Charge on the nucleus
- Shielding effect

The only option that does not influence ionization energy is atomic mass. Therefore, the number of factors that influence ionization energy is **3**.

### Summary of Factors:
- Size of the atom: Yes
- Charge on the nucleus: Yes
- Shielding effect: Yes
- Atomic mass: No

### Final Answer:
The number of factors that influence ionization energy is **3**.

---