Many style guides advise against apostrophes for any plurals.

The gymnasts need marks of 8s and 9s to qualify for the finals.

Others use an apostrophe and -s to form the plural of numbers, letters, symbols, and words referred to as terms.

The five Shakespeare’s in the essay were spelled five different ways.

Check your instructor’s preference. In any case, italicize numbers, letters, symbols, and terms but not the plural ending.