A bit confused about how promotions work...

Assuming most farmers retire at MX11/11A, does that mean most of them only get promoted 2-3 times in their life? And for the remaining years, they only get about $200 increment?

Or are there promotions within the grade? For e.g. a MX11 getting promoted to a new job title (and promotion increment) but still remains at MX11.

Grateful for any clarification!