There are a couple factors:
1. The song's BPM / speed, of course. The faster it is, the more likely it is to have higher...
2. Note density - how many notes on average there are per measure of the song; I.E. how densely packed together the hits are.
3. Technical grade - this includes things like enemy variety. Basically, how hard it is to sightread the hits.

What I usually do is cross-reference the base game's songs and see which song is the closest match to my level on the same Difficulty. This gives me a starting Intensity, which I can then tweak slightly based on how tough the chart actually is to play.

For an example, let's say that your Hard chart has similar note density and tech grade to Amalgamaniac. This gives you an Intensity of 17 to start with.
Then, through testing, you find out that the level actually came out a good bit tougher than Amalgamaniac. To compensate, you should bump the Intensity of the chart up a bit as well - say, to 19 - this brings in a few other songs to compare to in the base game.
Keep adjusting until you feel that the Intensity rating roughly matches what it would be if it were a base game song.

The maximum Intensity in the base game is 30 - the obvious example of which is Necro Sonatica on Impossible. You can set a higher value, though it would not affect the Intensity bar in the preview.

Hope this helps!

Was looking for this exact question. I see the game basically gauges intensity mainly off of enemy complexity rather than bpm and note density. Thanks for the breakdown Clone Fighter