Is it correct grammar to say twenty items or less?

No, it's not grammatically correct to say "twenty items or less."

Here's why:

* "Less" refers to quantities that cannot be counted (uncountable nouns). For example, "less water," "less sugar," or "less time."

* "Fewer" refers to quantities that can be counted (countable nouns). For example, "fewer items," "fewer people," or "fewer mistakes."

The correct way to phrase it is "twenty items or fewer."