**Hint:** To determine the number of significant figures in a given number, consider the rules for significant figures. Non-zero digits, zeros between non-zero digits, and trailing zeros after a decimal point are usually considered significant.

**Step-by-Step Solution:**

(i) 0.0025

This number has two significant figures (2 and 5).

(ii) 208

This number has three significant figures (2, 0, and 8).

(iii) 5005

This number has four significant figures (5, 0, 0, and 5).

(iv) 126,000

This number has three significant figures (1, 2, and 6). The trailing zeros, in this case, do not contribute to significant figures.

(v) 500.0

This number has four significant figures (5, 0, 0, and the zero after the decimal point).

(vi) 2.0034

This number has five significant figures (2, 0, 0, 3, and 4).

**Answer:**

(i) 2 significant figures

(ii) 3 significant figures

(iii) 4 significant figures

(iv) 3 significant figures

(v) 4 significant figures

(vi) 5 significant figures

**Note****:** Understanding the concept of significant figures is important in scientific and mathematical calculations. It helps in determining the precision of a measurement and how many decimal places should be retained in the result of a calculation. Counting significant figures is based on specific rules, as mentioned above, and can be useful in maintaining accuracy in scientific work.