Using apostrophes in single letters and numbers.
There are two a's in accommodation. Can you see how many r's and s's there are? Your 2's look like your 7's. All 1’s and 2’s. or All the 1s and 2s. Hawaii is spelt with two "i"s. Hawaii is spelt with two Is.

I recently received an email from someone that kindly pointed out an apostrophe error on my website: "Is there any chance you could correct the typos (misuse of the apostrophe in Tip 5). No apostrophe is needed in 2 rs. It is simply plural, not possessive."

This is the offending sentence: Can you see how many r's and s's there are? Yes, for many people this is confusing or just plain wrong. They want it to be written like this: Can you see how many rs and ss there are? Or Can you see how many Rs and Ss there are? Or Can you see how many "r"s and "s"s there are?

The grammar experts have this to say including myself in my Punctuation Guide and Workbook

Only use an apostrophe if there might be confusion, especially with single digit numbers and letters. All 1’s and 2’s.or All the 1s and 2s. 1000’s of people.or 1000s of people. Find all the number 7s. Do you write 1's and 7's as 1s or 7s? How many a's and m's are in accommodation?

So we can use an apostrophe in single letters and numbers for clarity.

This is taken from Grammar - one of my favourite sites.

Using Apostrophes in Unusual Plurals

The first thing to say about this topic is that apostrophes are not normally used to show plurals, and lots of your readers will hate it if you use an apostrophe for this purpose. However, that said, there are times when it helps to use an apostrophe to show a plural. For example:
* There are two i's in skiing.
* You use too many but's in your writing.

Of course, there are other ways of writing these. For example:
* There are two Is in skiing.
* You use too many "but"s in your writing.

The bottom line is apostrophes can be used in this way and for good reason — the apostrophe version is usually neater and clearer. However, be aware that you run the risk of annoying a high proportion of your readers if you do it.

I hope this clarifies things.

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