There are a variety of signs for "rich" depending on your meaning.
If you mean rich as in having a lot of money (monetarily rich) use the sign for
"rich" that is based on the concept of having a "pile of money":
https://youtu.be/4Uz8apATGXI
0:00 / 0:00
If discussing a desert you could fingerspell "rich" or (if you mean sweet) you could sign an exaggerated SWEET.
If you mean something has a lot of "fat and/or oil" in it you can use the sign for GRAVY https://youtu.be/QT8iaoWa-KQ or GREASE https://youtu.be/EP4yW8rnoZI (depending on your facial expression, context, and intent).
If you mean rich as in plentiful then sign PLENTY.
https://youtu.be/9TnUwUBcYzc
If you mean something is rich as in "thick" (such as a thick soup) then sign THICK
https://youtu.be/JFuewx99kL0
Also see: SPICY
https://youtu.be/ivKp9p_0InQ
https://youtu.be/X_AfZFQmrHM
Notes:
Notes: