It's not the peaks that make the loudness, it's the crest factor. Crest factor is the difference between the peak level and the RMS average level.
For setting the crest factor, first realize that master can reduce crest factor, but basically can't ever add it. Technically a mastering engineer could insert an expander to add crest factor, but an expander is going to make the mix sound very strange and won't do a good job of adding crest factor anyway.
The point is, you should send the largest crest factor to mastering that you can. Generally that means you should use no limiters or compressors on the stereo output bus. None! In fact, a good rule of thumb is to put nothing on the 2 bus, because processing the 2 bus the mastering engineer's job.
Streaming services like Spotify are now processing the music they stream by setting the average level, not the peak level. That means if you reduce the crest factor by too much before you send to Spotify, they will turn down your whole mix and you peaks will be made quieter. They are now recommending a crest factor of at least 14 dB when you deliver music to them. Most CD releases from the last 20 years have crest factors of 6 dB or less (that means they are 8 dB louder than the Spotify recommendation). Spotify wants your stuff to be quiet! Some online services are benchmarking average levels at -18 dBFS, which means you'll want a crest factor of 18 dB. And that's after mastering!
So, you probably want to deliver a crest factor to mastering of at least 20 dB. Mixing to a 20 dB crest factor means you can more easily put your music with film or video, which has been standardized at 20 dB crest factors since the 90s.
Just to be on the safe side, your peak level probably shouldn't be above -3 dBFS. That means when you're metering your mix, you should see RMS average levels around -23 dBFS and peaks not greater than -3 dBFS.
Beyond all that:
Ask your mastering engineer what you should deliver to them!