Bob - I would suggest 31 is too many if you are just getting started. You will get various forms of the same words/phrases by accident.
Easiest way to think is - what would you type in to find you.
Example - Suppose you sell specialist shoes for the pole vault ...(just suppose)
You wouldn't type in "shoes" - Too generic (not a BUYING keyword)
You WOULD type in say "asics PV2000 shoe" - That IS a buying keyword.
See the difference.