The correct answer is '3' - there are three silver-containing compounds among the given options. Let's analyze each reagent or compound to understand why this is the correct answer.

Nessler's Reagent:
- Nessler's reagent is a chemical solution commonly used to test for the presence of ammonia.
- It does not contain silver, so it is not a silver-containing compound.

Tollens' Reagent:
- Tollens' reagent is a colorless solution that is used to test for the presence of aldehydes.
- It contains silver nitrate (AgNO3), which reacts with aldehydes to produce a silver mirror.
- Therefore, Tollens' reagent is a silver-containing compound.

Baeyer's Reagent:
- Baeyer's reagent is a solution of potassium permanganate (KMnO4) in sulfuric acid.
- It does not contain silver, so it is not a silver-containing compound.

Corrosive Sublimate:
- Corrosive sublimate is a common name for mercury(II) chloride (HgCl2).
- It contains mercury, not silver, so it is not a silver-containing compound.

Horn Silver:
- Horn silver is a mineral form of silver chloride (AgCl).
- It contains silver, so it is a silver-containing compound.

Ruby Silver:
- Ruby silver is not a specific compound or reagent.
- It may refer to a variety of silver minerals or silver compounds that have a reddish color, but without further context, it is difficult to determine a specific compound.
- Therefore, we cannot definitively classify it as a silver-containing compound.

Calomel:
- Calomel is a common name for mercury(I) chloride (Hg2Cl2).
- It contains mercury, not silver, so it is not a silver-containing compound.

Summary:
Based on the analysis above, we can conclude that there are three silver-containing compounds among the given options: Tollens' reagent, Horn silver (silver chloride), and potentially Ruby silver (depending on the specific compound referred to).