For 20 servings, Thomas needs 20*1 = <<20*1=20>>20 cups of chickpeas.
To have 20 cups, Thomas needs 20*6 = <<20*6=120>>120 ounces.
120 ounces/16 ounces = <<120/16=7.5>>7.5 cans of chickpeas.
Since Thomas cannot buy half a can, he needs to buy 8 cans.
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