Can I say "under 50 words"?

A little morei

Words are countable — so fewer than is the rule-abiding choice, not less than and not under. Under is spatial at root; editors who notice tend to rewrite it for count contexts. For a word limit, where the count is the whole point, "fewer than 50 words" is the tighter form. Style guides differ; house style usually wins.

The full entryii

More quick answersiii

• → When do I use "fewer"? Use fewer for things you can count: fewer chairs, fewer errors, fewer hours logged.
• → When do I use "less"? Use less for amounts you measure rather than count: less water, less noise, less patience.
• → Is it "less people" or "fewer people"? "Fewer people." People are countable, so the correct word is fewer.
• → Is it "10 items or less" or "10 items or fewer"? Grammatically, "10 items or fewer." Items are countable, so they take fewer.
• → Is it "less calories" or "fewer calories"? "Fewer calories." A calorie is a unit you can count, so the right word is fewer.