Hello
The square brackets [ ] relate to the different sizes so yours is the smallest and given first, before the square brackets.
The curved brackets ( ) mean do all this and do it the number of times which is given outside, after the closing bracket. For you this is 11.
So, for smallest size
Rib 0, just ignore
(rib 1, work 2 tog, rib 3, work 2 tog, rib 1) do all this 11 times
Rib 0, ignore
77 stitches
You have not said how many stitches you have before this section but based on the number of stitches that you are instructed to work here I believe you are starting with 99.
If you are not starting with 99 then we should rethink what I’ve said above.
Hope this helps. If it’s not clear please say so and ask any more questions you need to.