Solution:

Corrosion is a process in which a metal is converted into its ionic form due to the reaction with the environment. In this case, we want to prevent the corrosion of Ni to Ni2+ by using other metals.

To prevent the corrosion of Ni, we need to use a metal that has a higher reduction potential (E value) than Ni. The metals with higher E values will be able to donate electrons to Ni and prevent it from getting oxidized.

Let's compare the E values of Ni2+/Ni (-0.26 V) with the E values of other metals given in the question.

Metals with higher E values than Ni

The following metals have higher E values than Ni and can be used to prevent the corrosion of Ni.

1. Ag/Ag+ (0.80 V): This metal has the highest E value among all the metals given in the question. It can easily donate electrons to Ni and prevent its oxidation.

2. Cu/Cu2+ (0.34 V): This metal can also donate electrons to Ni and prevent its oxidation. However, its E value is lower than Ag.

3. Pt(H2)/H+ (0.00 V): This metal has the same E value as Ni. It may prevent the corrosion of Ni, but it is not a very effective option.

Metals with lower E values than Ni

The following metals have lower E values than Ni and cannot be used to prevent the corrosion of Ni.

1. Fe2+/Fe (-0.44 V): This metal has a lower E value than Ni and cannot prevent its oxidation.

2. Zn2+/Zn (-0.76 V): This metal has a much lower E value than Ni and cannot prevent its oxidation.

3. AI3+/AI (-1.66 V): This metal has a very low E value and cannot prevent the corrosion of Ni.

4. Mg2+/Mg (-2.37 V): This metal has the lowest E value among all the metals given in the question. It cannot donate electrons to Ni and prevent its oxidation.

Therefore, we can conclude that 3 metals (Ag, Cu, Pt) can be used to prevent the corrosion of Ni to Ni2+. However, the question asks for how many metals can be used. In this case, we need to consider the practicality of using these metals. Ag and Pt are expensive and not very practical options for preventing corrosion. Therefore, we can only consider Cu as a practical option for preventing the corrosion of Ni. Hence, the answer is 4 (Cu, Ag, Pt, and Cu).