31 spteps have more info and accuracy but they take longer to evaluate the data. 21 steps are enough, but you might want to use the step wedges for other tests and then 31 steps might be needed. So, I recommend a 31-step test tablet in the size of your negative. To save shipping costs, and you need one anyway some day. Also get a 31-step version in 4x5 for contact printing. Don't get the calibrated version of any step tablet. They are terribly expensive and all they offer is a list of accurately measured densities. You don't need that. get all test tablets as non-calibrated.
Hope this helps.