The reason you have found different answers is because this is a matter of style, not a rule, and different style guides will give different guidance.
For instance, here is what The Chicago Manual of Style (17th ed.), 6.116, says about the apostrophe:
The apostrophe has three main uses: to indicate the possessive case, to stand in for missing letters or numerals, and—in rare instances—to form the plural of certain expressions.
And in 7.15:
Capital letters used as words, numerals used as nouns, and abbreviations usually form the plural by adding s. To aid comprehension, lowercase letters form the plural with an apostrophe and an s (compare “two as in llama” with “two a’s in llama”).
the three Rs
x’s and y’s
The Associated Press says the following about apostrophes beyond the simple posessive:
OMITTED LETTERS: I've, it's, don't, rock 'n' roll, 'tis the season to be jolly. He is a ne'er-do-well.
OMITTED FIGURES: The class of '62. The Spirit of '76. The '20s.
PLURALS OF A SINGLE LETTER: Mind your p's and q's. He learned the three R's and brought home a report card with four A's and two B's. The Oakland A's won the pennant.
DO NOT USE: For plurals of numerals or multiple-letter combinations.
Notice the conflicting guidance between the two when it comes to single capital letters.
However, according to both of these guides (which, together, are the most authoritative, in North America anyway), there would be no apostrophe for your number:
Four twos make an eight.
But you should follow whichever style guide you ascribe to.
Note, however, that this guidance has changed over time. About thirty years ago, it used to be that the apostrophe was used to make acronyms plural. We used to write DVD's, for example. That's something that most people no longer do.