How many decibels is twice as loud?

The decibel is a power ratio. It can however be used to describe non-power ratios like voltage or SPL. 3dB is a doubling of power. 6dB is a doubling of voltage or SPL.

6dB is twice as loud therefore. 20dB is 10x as loud.^[ Decibels (dB): The Only 5 Things You (Probably) Need to Know]