Answer
$1L=1000mL$
Work Step by Step
The abbreviation m stands for the prefix milli, which is $10^{-3}$ of the base unit.
When the base unit is liters,
$0.001L=1mL\longrightarrow$ multiply each side by 1000
$0.001L\times1000=1mL\times1000$
$1L=1000mL$