If you are not sure what target loudness you should be aiming for, when mastering a song for streaming services, go for an integrated -14 LUFS or above. 

Even if you are louder than -14 LUFS, do not worry since your song will get turned down automatically. An integrated – 14 LUFS is the default loudness normalization setting of the most popular streaming services.

If you want to go above -14 LUFS for streaming services that do not have normalization, watch at what level other songs are set up and go for the same thing. 

But the thing you should never do when mastering for any streaming service is to go above – 1dB True Peak because your audio might start clipping!

If you want to find out what are the official loudness recommendations for different streaming services, look at this table here.