Introduction:
Amphoteric oxides are oxides that can react with both acids and bases. They can act as either an acid or a base depending on the conditions. In this question, we are given a list of elements and we need to determine how many of them form amphoteric oxides.

Explanation:
To determine which elements form amphoteric oxides, we need to analyze their electronic configurations and properties.

1. Silicon (Si):
Silicon forms silicon dioxide (SiO2), which is an acidic oxide. It reacts with bases to form silicates but does not react with acids. Therefore, Si does not form an amphoteric oxide.

2. Phosphorus (P):
Phosphorus forms phosphorus pentoxide (P2O5), which is an acidic oxide. It reacts with bases to form phosphates and with acids to form oxyacids. Therefore, P forms an amphoteric oxide.

3. Nitrogen (N):
Nitrogen forms nitrogen dioxide (NO2), which is an acidic oxide. It reacts with bases to form nitrates but does not react with acids. Therefore, N does not form an amphoteric oxide.

4. Boron (B):
Boron forms boron trioxide (B2O3), which is an acidic oxide. It reacts with bases to form borates but does not react with acids. Therefore, B does not form an amphoteric oxide.

5. Tin (Sn):
Tin forms tin(II) oxide (SnO) and tin(IV) oxide (SnO2). Both of these oxides are amphoteric and can react with both acids and bases. Therefore, Sn forms an amphoteric oxide.

6. Lead (Pb):
Lead forms lead(II) oxide (PbO) and lead(IV) oxide (PbO2). Both of these oxides are amphoteric and can react with both acids and bases. Therefore, Pb forms an amphoteric oxide.

7. Oxygen (O):
Oxygen itself is not an amphoteric oxide, as it generally acts as a base and reacts with acids to form oxoanions. However, it is important to note that oxygen can form amphoteric oxides when combined with certain elements, such as Sn and Pb.

Conclusion:
Out of the given elements, Phosphorus (P), Tin (Sn), and Lead (Pb) form amphoteric oxides. These elements have oxides that can react with both acids and bases, making them capable of acting as either an acid or a base depending on the conditions.