When do you put the number 2 on your Non-Commissioned Officer ribbon, after completion of the course or after you get pinned?

Let the final phase 1059 be your guide...

Upon completion of ALC and any DL requirements.
Rank has nothing to do with it.

After all phases of your ALC are completed

It's based on the graduation from NCO PD courses, not the wearing of the rank, so if you are an ALC grad you wear the numeral 2. See the reference below.
https://www.hrc.army.mil/content/NCOPDR%20changes

After you complete ALC

Why is there a NCO ribbon?. No wonder you see people with three or four rows of ribbon and none of them relate to Combat.

I wasn't able to have a numeral 2 on my 214 because I only did the MOS portion, not the "Phase 1" portion that was online at the time. I tried from 2010 until my discharge in 2012 to get enrolled.

I have no idea what ALC is, but I put my 2 on right after graduation of BNCOC.

Complete course and graduate

After you graduate the course.