The rule of thumb: pick up 2 rib st for every 3 body-side st.
However, that said...I have also picked up 3 for 4.
Ribbing is very tricky. I've had to tear down ribbing because it was ripply (too many st) or because it was "bowed" (too few stitches).
I am always quick to "re-do" wonky ribbing to get it right!