Following your most important ECs, does it look better to fill the remaining spots (I’ve got 3 left) with clubs I’m in but just not super involved (like math and Spanish honor society, in which I participate but are nothing special)? Or would leaving it blank be better, so they admissions officers don’t have to read a few extra lines?
If you have space for additional ECs, then include them.
You could argue either way.
The other argument would be that listing less important activities dilutes the impact of your more significant activities. S listed only the most significant activities- the ones he would like to discuss at an interview- and left a blank (listed 9 instead of 10), rather than fill it in with a low-commitment, occasional activity. He eliminated from mention altogether the clubs he had tried in 9th or 10th grade but abandoned by 11th. Moreover, he chunked some related activities onto one line and explained them as one interest in the line where you describe them.
You should include whatever matters to you and that you think adds to the picture of who you are.