Answer
$\bf\text{Exact numbers}$ are known with complete certainty, and are considered to have $\bf\text{infinite significant figures}$
Work Step by Step
If we multiply
$123 * 1,100,000 = 135300000 = 1.4 * 10^8$
our answer would only have two significant figures because $1,100,000$ only has two significant figures.
Some numbers are $\bf\text{Exact}$ meaning they have infinite significant figures in calculations.
Examples of exact numbers:
1 dozen= 12
12 inches = 1 foot
2.54cm = 1 inch
1m = 1000mm
When we do calculations with exact numbers we are only limited by the significant figures in the other numbers in our equation. Ex: $2.54$ is actually $2.5400000000000...$
$3,456,789in*\dfrac{2.54cm}{1in} = 8,780,244cm$
