How many milliliters are equivalent to 2.7 liters?

$2700 \cdot m L$
The $\text{m}$ prefix in $m L$ means $\text{milli}$, i.e. $\times {10}^{-} 3$.
And thus $\frac{1 \cdot L}{1 \cdot m L} = \frac{1 \cdot \cancel{L}}{1 \cdot \times {10}^{-} 3 \cancel{L}} = \frac{1}{{10}^{-} 3} = \frac{1}{\frac{1}{1000}} = 1000$, i.e. a ${10}^{3}$ factor as required.
Note the arithmetic, $\frac{b}{a} ^ - 1 = \frac{b}{\frac{1}{a}} = a \times b$, this is an important result for such dimensional analysis, as you can see in the given example.