How many following Ammonium salts will evolve `N_(2)` gas on heating?
`(NH_(4))_(2)CO_(3),(NH_(4))_(2)Cr_(2)O_(7),NH_(4)NO_(2),NH_(4)ClO_(4),NH_(4)Cl,(NH_(4))_(2)S,(NH_(4))_(2)C_(2)O_(4)`

Explanation

To determine how many of the given ammonium salts will evolve nitrogen gas (N₂) upon heating, we need to analyze each salt based on the anions present in them. The key point to remember is that ammonium salts will evolve nitrogen gas if they contain the anions NO₂⁻, NO₃⁻, or Cr₂O₇²⁻.

Let's analyze each salt one by one:

1. **(NH₄)₂CO₃ (Ammonium Carbonate)**:
- Anion: CO₃²⁻ (Carbonate)
- Upon heating, this salt decomposes to form ammonia (NH₃) and carbon dioxide (CO₂), not nitrogen gas.
- **Conclusion**: Does not evolve N₂.

2. **(NH₄)₂Cr₂O₇ (Ammonium Dichromate)**:
- Anion: Cr₂O₇²⁻ (Dichromate)
- Upon heating, this salt decomposes to evolve nitrogen gas (N₂).
- **Conclusion**: Evolves N₂.

3. **NH₄NO₂ (Ammonium Nitrite)**:
- Anion: NO₂⁻ (Nitrite)
- Upon heating, this salt decomposes to form nitrogen gas (N₂) and water (H₂O).
- **Conclusion**: Evolves N₂.

4. **NH₄ClO₄ (Ammonium Perchlorate)**:
- Anion: ClO₄⁻ (Perchlorate)
- Upon heating, this salt decomposes and can also produce nitrogen gas (N₂).
- **Conclusion**: Evolves N₂.

5. **NH₄Cl (Ammonium Chloride)**:
- Anion: Cl⁻ (Chloride)
- Upon heating, this salt does not produce nitrogen gas; it produces ammonia (NH₃) and hydrogen chloride (HCl).
- **Conclusion**: Does not evolve N₂.

6. **(NH₄)₂S (Ammonium Sulfide)**:
- Anion: S²⁻ (Sulfide)
- Upon heating, this salt does not produce nitrogen gas; it produces ammonia (NH₃) and hydrogen sulfide (H₂S).
- **Conclusion**: Does not evolve N₂.

7. **(NH₄)₂C₂O₄ (Ammonium Oxalate)**:
- Anion: C₂O₄²⁻ (Oxalate)
- Upon heating, this salt decomposes to produce ammonia (NH₃) and carbon monoxide (CO), not nitrogen gas.
- **Conclusion**: Does not evolve N₂.

Now, let's summarize the findings:
- The salts that evolve nitrogen gas (N₂) upon heating are:
- (NH₄)₂Cr₂O₇
- NH₄NO₂
- NH₄ClO₄

Thus, the total number of ammonium salts that will evolve N₂ gas on heating is **3**.

### Final Answer:
**3**