How do I know how much volume is right?

I am a newbie user of Audacity and I digitalize cassette tapes that contain old radio programs. I have bougth a
good DENON tape player and I don’t get much hiss, buzz or any cicks. When I record from the cassette to computer
I turn down the output from the recorder when the tape plays so that I get very low peaks. When the tape is finished
I select all and do Effect > Amplify with custom settings to increase the volume I use default settings that Audacity

I have some questions about this:

What is the Effect > Amplify amplification based on? I thought that it will increase the peaks of the volume
until one peak reaches max volume. Sometimes i do an amplification that doesn’t reach what seems to be the the
max volume. After amplification no volume peak reaches the outer outer edge of the recording only at 50%.
How could that be?

Sometime a tape has parts in different volume like talk in low volume and music in high volume.
When i amplify everything to max and the music reaches max recommended volume the talk still has to low volume.
How do you recommend me to handle this?

I will do mp3:s that should be played on different systems, YouTube etc. How can I get a good “standardized” volume
that is about “right” and is never for example to low on max volume for the majority of listeners. So that a user
can comfortably listen to mp3:s created from many different tape without having to turn up to max volume and still
barely be able to hear anything or hurting his ears due to big volume?

Do you have any suggestions?

You thought right. The default amplification amount brings the highest (or most ‘negative’) peak to “0 dB” (the top, or bottom of the track).

Almost certainly there is a small “spike” (peak) in the recording that is higher than the rest of the recording. This is quite a common problem when recording vinyl as there can be a click or crackle much higher than the rest of the track. Om a long track it can be hard to spot a single high “click”. For recordings from tape, the most likely place for such a click is at the very start or very end of the recording - zoom in and look carefully at those regions (zooming: Zooming Overview - Audacity Manual)

If you still can’t find the click, try the amplify effect again, but select the “allow clipping” checkbox, then amplify to +0.1 dB. Then in the “View” menu, ensure that “show clipping” is enabled. This will give a red vertical line at each point where the waveform exceeds 0 dB. Fix those clicks, then amplify again with “allow clipping” turned off (NOT selected). That will bring the entire track back within the ‘legal’ 0 dB range.

Use the “Envelope tool” to bring down the loud parts to the same level as the quiet parts.
When you are happy that it sounds “right” (even though the overall level is a bit quiet), use “Tracks >Mix and Render” to “fix” (“apply”) the envelope permanently to the audio. Then use the Amplify or Normalize effect to bring the overall level up.

Home recordings will usually sound a bit quieter than modern commercial MP3 recordings - that’s because many commercial MP3 recordings are TOO LOUD.
For best sound quality, don’t be tempted to compete. Get it sounding good, then Normalize to about -2 dB and export it.
if “loud” is more important than “quality” (shudder), Amplify to the max, then use the Limiter effect with Makeup Gain to make it a bit louder (Limiter - Audacity Manual), then export.

Interesting article about the loudness of modern recordings: