For mastering music you probably want to start with a compression ratio of “1.5:1” doing a gain reduction of about say 3 dB or maybe 2 dB depending on the program material you are mastering.
The reason for using a compression ratio of 1.5:1 as opposed to a compression ratio of 4:1 or 10:1 is because mastering is fairly a delicate process and you don’t want to mangle those dynamics before the music even hits a brickwall limiter.
If you use high compression ratios you will suck the soul out of the music and the final result can sound like a constant drone of undynamic but a young pretender in the loudness wars. However, if that’s the sound you want then the choice is all yours.
Alternative Mastering Compression Ratio?
Be aware that not all compressors are designed with the same mindset. Some compressors don’t even give you the option to select a compression ratio of 1.5:1 but instead you are left with a choice of compression ratios such as 2:1, 4:1 and 10:1 and so forth.
If that’s the case then you should lean towards the softest compression ratio you can select e.g 2:1 before you try those high compression ratios.
One of the advantages of using a soft compression ratio like 2:1 is based on the ideal need to retain the maximum possible punch of the music since high compression ratios will attenuate those transients mercilessly.