I'd interpret TABSTOP 5 8 10 to mean there are tab stops at the 5th, 8th, and 10th columns (and after that, every 8 columns, or whatever you're using as the default. It's open to question whether the next tab stop after column 10 should be at column 18 (8 spaces later) or 16 (the next multiple of the default 8).
A list of tab stops specifies particular columns, not distances between stops. That is, if the list is 5,8,10 then a tab in positions 1-4 should place the cursor at 5, in positions 5-7 should place the cursor at 8, and 8-9 place the cursor at 10. After each newline the argument list should start over from the first tab stop again. The behavior on a line after the last defined tab stop is up to you, typically you would go back to some default tab stop interval.
You have some leeway here, as you're designing it; however, the most popular method is to continue with the default width after the last supplied width.
For example, if [5, 8, 10] is supplied and the default is 8, it would continue as [5, 8, 10, 18, 26, 34, ...] or [5, 8, 10, 16, 24, 32, ...], depending on preference.
Note that I'm using these numbers as tab stops, instead of widths. So [5, 8] means the first stop is at 5 with width 5, and the second is at 8 with width 3.
What if for example list of arguments is 5, 10.
I enter abc and then a newline.
Should i switch to next argument then? to 10? For next tab stop? Or stay on the current TAB_STOP until it has been 'used'?