A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
The number of "selection handles" seems to be maximised at 16. Any chart with more than 16 data points will only display 16 such handles. I've tried on PC and Mac with the same results. Created a line chart with 100 data points and only 16 handles are displayed somewhat evenly spread on the line.
I searched the web but found no documentation regarding this fact but it seems to make sense as you would not want to see a handle on each and every data point when you click a series. Consider the "max 16 handles" as a visual reference that merely indicate which series you have selected for formatting.