Your mix might already hit 0 dB on the loudest peak — but loudness isn't about the single highest peak, it's about the average energy over time. Commercial masters use compression and limiting to bring the whole track up close to the peak ceiling, so the average (LUFS) is much higher. That's what your ear hears as "loud."
If you just add gain, the peaks hit the ceiling and clip (distort) long before the average gets loud enough. Mastering raises the average without clipping by controlling peaks first — soft saturation, compression, and a true-peak limiter.
Past a point, more loudness crushes the life out of a track, and streaming platforms normalize playback loudness anyway. The real goal is competitive loudness for your genre with the right dynamics and a safe true-peak ceiling (−1 dBTP).
It depends on genre, not a single number. Aim for your genre's commercial range and keep true peak ≤ −1 dBTP. See how to get a Spotify-ready master.
No — beyond a point it sounds worse, and streaming normalizes loudness anyway.