I also seem to be short 2 of these. As well as 2 x 3/4" nylon spacers.
I just counted what I used, and there are definitely 20 in use, which is what the bag says.
8 - for mounting the base plates to the lower Makerslide sections
2 - for mounting the delrin nut
4 - for mounting the rotary tool mounts to the Z axis Makerslide
4 - for mounting the x axis Makerslide to the gantry sides
2 - for the y axis belt clips
But I'm missing 2 for the x axis belt clips
I'll just grab some at Lowe's, but might need to update the BOM for the next run. The BOM seems ok for the spacers, it looks like I was just shorted 2 of them.