At 22" across and 12" deep it would be a true 20gal, and a very large pot if you're fertigating with water soluble nutrients. So that's a biggest consideration, how you're feeding and through what media.
As a general recommendation I'd say one plant and make it a high production, hardy disease tolerant cherry, like Sungold or Sweet Million.
If it's a more confident grower wanting to explore pruning, 2 plants trimmed to one stem, like zipcode said.
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